Meson cleanup

This removes some macro definitions:

• _REENTRANT and _THREAD_SAFE were taken from the VLC build system and have since been removed there, so it seems safe to remove those here as well.
• _POSIX_PTHREAD_SEMANTICS is required for POSIX thread flavor of threading functions but instead we can define _POSIX_C_SOURCE to a value higher than 199506L, when including <pthread.h>, see https://docs.oracle.com/cd/E19455-01/806-5257/compile-3/index.html
• __EXTENSIONS__ is used to enable Solaris extensions, as far as I know we are not using any.
• _FILE_OFFSET_BITS is already defined to 64 by meson
• _ISOC99_SOURCE should not be required as we do not support compiling with -std c89
• _ISOC11_SOURCE should not be required as we set -std c11, so we do not need any C11 extensions to C99 as we already use C11.
• _POSIX_SOURCE is not required as we already set _POSIX_C_SOURCE to a positive integer.
• _XOPEN_SOURCE should not be needed as we already have _POSIX_C_SOURCE.
• _XOPEN_SOURCE_EXTENDED should not be required as we do not need any X/Open Unix extensions to my knowledge.
• _BSD_SOURCE should not be required as we already set _POSIX_C_SOURCE.
• _SVID_SOURCE should not be required as we already set _POSIX_C_SOURCE.

A follow-up commit sets _POSIX_C_SOURCE to 200112L to indicate we use POSIX.1–2001.