https://bugs.gentoo.org/577238

we should be able to drop this with gcc-6+

--- a/libstdc++-v3/config/locale/generic/c_locale.h
+++ b/libstdc++-v3/config/locale/generic/c_locale.h
@@ -70,7 +70,7 @@
     __builtin_va_list __args;
     __builtin_va_start(__args, __fmt);
 
-#ifdef _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
     const int __ret = __builtin_vsnprintf(__out, __size, __fmt, __args);
 #else
     const int __ret = __builtin_vsprintf(__out, __fmt, __args);
--- a/libstdc++-v3/config/locale/gnu/c_locale.h
+++ b/libstdc++-v3/config/locale/gnu/c_locale.h
@@ -88,7 +88,7 @@
     __builtin_va_list __args;
     __builtin_va_start(__args, __fmt);
 
-#ifdef _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
     const int __ret = __builtin_vsnprintf(__out, __size, __fmt, __args);
 #else
     const int __ret = __builtin_vsprintf(__out, __fmt, __args);
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -5246,7 +5246,7 @@
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
 
-#if __cplusplus >= 201103L && defined(_GLIBCXX_USE_C99)
+#if __cplusplus >= 201103L && (defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__))
 
 #include <ext/string_conversions.h>
 
--- a/libstdc++-v3/include/bits/locale_facets.tcc
+++ b/libstdc++-v3/include/bits/locale_facets.tcc
@@ -992,7 +992,7 @@
 	char __fbuf[16];
 	__num_base::_S_format_float(__io, __fbuf, __mod);
 
-#ifdef _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
 	// Precision is always used except for hexfloat format.
 	const bool __use_prec =
 	  (__io.flags() & ios_base::floatfield) != ios_base::floatfield;
--- a/libstdc++-v3/include/bits/locale_facets_nonio.tcc
+++ b/libstdc++-v3/include/bits/locale_facets_nonio.tcc
@@ -578,7 +578,7 @@
     {
       const locale __loc = __io.getloc();
       const ctype<_CharT>& __ctype = use_facet<ctype<_CharT> >(__loc);
-#ifdef _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
       // First try a buffer perhaps big enough.
       int __cs_size = 64;
       char* __cs = static_cast<char*>(__builtin_alloca(__cs_size));
@@ -751,7 +751,7 @@
 		case 'S':
 		  // Seconds. [tm_sec]
 		  // [00, 60] in C99 (one leap-second), [00, 61] in C89.
-#ifdef _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
 		  __beg = _M_extract_num(__beg, __end, __tm->tm_sec, 0, 60, 2,
 #else
 		  __beg = _M_extract_num(__beg, __end, __tm->tm_sec, 0, 61, 2,
--- a/libstdc++-v3/include/c_compatibility/math.h
+++ b/libstdc++-v3/include/c_compatibility/math.h
@@ -56,7 +56,7 @@
 using std::floor;
 using std::fmod;
 
-#if _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
 using std::fpclassify;
 using std::isfinite;
 using std::isinf;
--- a/libstdc++-v3/include/c_compatibility/wchar.h
+++ b/libstdc++-v3/include/c_compatibility/wchar.h
@@ -103,7 +103,7 @@
 using std::wmemset;
 using std::wcsftime;
 
-#if _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
 using std::wcstold;
 using std::wcstoll;
 using std::wcstoull;
--- a/libstdc++-v3/include/c_global/cstdio
+++ b/libstdc++-v3/include/c_global/cstdio
@@ -146,7 +146,7 @@
   using ::vsprintf;
 } // namespace
 
-#if _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
 
 #undef snprintf
 #undef vfscanf
--- a/libstdc++-v3/include/c_global/cstdlib
+++ b/libstdc++-v3/include/c_global/cstdlib
@@ -195,7 +195,7 @@
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
 
-#if _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
 
 #undef _Exit
 #undef llabs
--- a/libstdc++-v3/include/c_global/cwchar
+++ b/libstdc++-v3/include/c_global/cwchar
@@ -232,7 +232,7 @@
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
 
-#if _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
 
 #undef wcstold
 #undef wcstoll
@@ -289,7 +289,7 @@
   using std::vwscanf;
 #endif
 
-#if _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
   using std::wcstold;
   using std::wcstoll;
   using std::wcstoull;
--- a/libstdc++-v3/include/c_std/cstdio
+++ b/libstdc++-v3/include/c_std/cstdio
@@ -144,7 +144,7 @@
   using ::vsprintf;
 } // namespace std
 
-#if _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
 
 #undef snprintf
 #undef vfscanf
--- a/libstdc++-v3/include/c_std/cstdlib
+++ b/libstdc++-v3/include/c_std/cstdlib
@@ -192,7 +192,7 @@
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
 
-#if _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
 
 #undef _Exit
 #undef llabs
--- a/libstdc++-v3/include/c_std/cwchar
+++ b/libstdc++-v3/include/c_std/cwchar
@@ -228,7 +228,7 @@
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
 
-#if _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
 
 #undef wcstold
 #undef wcstoll
--- a/libstdc++-v3/include/ext/vstring.h
+++ b/libstdc++-v3/include/ext/vstring.h
@@ -2680,7 +2680,7 @@
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
 
-#if ((__cplusplus >= 201103L) && defined(_GLIBCXX_USE_C99))
+#if ((__cplusplus >= 201103L) && (defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)))
 
 #include <ext/string_conversions.h>
 
--- a/libstdc++-v3/include/tr1/cstdio
+++ b/libstdc++-v3/include/tr1/cstdio
@@ -33,7 +33,7 @@
 
 #include <cstdio>
 
-#if _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
 
 namespace std _GLIBCXX_VISIBILITY(default)
 {
--- a/libstdc++-v3/include/tr1/cstdlib
+++ b/libstdc++-v3/include/tr1/cstdlib
@@ -35,7 +35,7 @@
 
 #if _GLIBCXX_HOSTED
 
-#if _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
 
 namespace std _GLIBCXX_VISIBILITY(default)
 {
--- a/libstdc++-v3/include/tr1/cwchar
+++ b/libstdc++-v3/include/tr1/cwchar
@@ -52,7 +52,7 @@
   using std::vwscanf;
 #endif
 
-#if _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
   using std::wcstold;
   using std::wcstoll;
   using std::wcstoull;
--- a/libstdc++-v3/include/tr1/stdlib.h
+++ b/libstdc++-v3/include/tr1/stdlib.h
@@ -33,7 +33,7 @@
 
 #if _GLIBCXX_HOSTED
 
-#if _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
 
 using std::tr1::atoll;
 using std::tr1::strtoll;
--- a/libstdc++-v3/src/c++11/debug.cc
+++ b/libstdc++-v3/src/c++11/debug.cc
@@ -788,7 +788,7 @@
 				     int __n __attribute__ ((__unused__)), 
 				     const char* __fmt, _Tp __s) const throw ()
     {
-#ifdef _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
       std::snprintf(__buf, __n, __fmt, __s);
 #else
       std::sprintf(__buf, __fmt, __s);
--- a/libstdc++-v3/src/c++98/locale_facets.cc
+++ b/libstdc++-v3/src/c++98/locale_facets.cc
@@ -72,7 +72,7 @@
 
     ios_base::fmtflags __fltfield = __flags & ios_base::floatfield;
 
-#ifdef _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
     // Precision is always used except for hexfloat format.
     if (__fltfield != (ios_base::fixed | ios_base::scientific))
 #endif
@@ -89,7 +89,7 @@
       *__fptr++ = 'f';
     else if (__fltfield == ios_base::scientific)
       *__fptr++ = (__flags & ios_base::uppercase) ? 'E' : 'e';
-#ifdef _GLIBCXX_USE_C99
+#if defined(_GLIBCXX_USE_C99) || defined(__UCLIBC__)
     else if (__fltfield == (ios_base::fixed | ios_base::scientific))
       *__fptr++ = (__flags & ios_base::uppercase) ? 'A' : 'a';
 #endif