Marvin Scholz authored Sep 23, 2018 and Jean-Baptiste Kempf committed Sep 24, 2018

- _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.