configure: Refactor setting of feature test macro _XOPEN_SOURCE

Some C libraries, eg glibc, uclibc, and musl, uses feature test macros
to expose definitions conforming to the standards ISO C, POSIX and
extensions. According to which feature test macros are defined by the
user or the compiler, a header file, eg <features.h>, used by these
libraries internally defines various other macros.

glibc and uclibc also defines release test macros, eg __GLIBC__ and
__UCLIBC__ in <features.h>. musl does not have (and does not want) a
macro __MUSL__. Therefore it is not possible to check for the musl
library.

However, building FFmpeg with musl needs the feature test macro
_XOPEN_SOURCE=600 to be defined.

Signed-off-by: Jörg Krause <jkrause@posteo.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

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