build: Define both _POSIX_C_SOURCE and _XOPEN_SOURCE with versions

FreeBSD libc assumes defining _POSIX_C_SOURCE without specifying a
version means a pre-C89 environment with the oldest version of POSIX
standard, IEEE Std 1003.1-1988. Defining _POSIX_C_SOURCE without
defining _XOPEN_SOURCE also causes XSI to be disabled. However, M_PI
constant is only available in XSI. Therefore, we have to define
_XOPEN_SOURCE if we want to define _POSIX_C_SOURCE.

https://bugzilla.gnome.org/show_bug.cgi?id=783442