M_PI not available with gcc --std=c11 but with --std=gnu11?

I noticed M_PI is unavailable on c11. By looking at /usr/include/math.h I can see M_PI is defined if:

#if !defined(__STRICT_ANSI__) || ((_XOPEN_SOURCE - 0) >= 500)
...
#define M_PI 3.1415...
#endif 

Moreover in the math.h from glibc __STRICT_ANSI__ is replaced with __USE_MISC. I am completely lost with this.

What is the story in between --std=c11 and the constants defined in math.h?

Which libc should I consider on a debian distribution ?