configure: add _GNU_SOURCE to CFLAGS by default

In theory, projects have to define feature test macros to enable various
system functionality in system headers. (This is done so to ensure new
identifiers can be added to system headers, without breaking old
programs by causing name conflicts.) This includes macros like
_GNU_SOURCE, _BSD_SOURCE, _POSIX_C_SOURCE etc.

Traditionally, gcc as well as glibc headers implicitly assumed
_GNU_SOURCE if no feature test macros were defined by the user.
clang did this too to ensure compatibility with gcc centric programs
(which in practice includes most Linux programs).

However, it appears recent clang versions started to prefer BSD
traditional function over the POSIX, which switches the definition
of a function used by mp_msg.c:

       pid_t getpgrp(void);                 /* POSIX.1 version */
       pid_t getpgrp(pid_t pid);            /* BSD version */

mp_msg.c expects the POSIX version, while clang gives us the BSD
version, resulting in a compilation failure.

Solve this by defining _GNU_SOURCE. This requests most features from
system headers, and explicitly prefers POSIX definitions over BSD,
which should fix the compilation issue.

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