Sadly most versions still cannot properly handle the suppression
via `_Pragma`, so it has to leak to the users when they use older
GCC versions to compile their code
C++11 math requires _GLIBCXX_USE_C99_MATH_TR1 to be true with gcc/clang.
Also fixes an issue with uClibc-ng where __UCLIBC__ is defined in features.h but
that is not included here and is thus no-op.
