Build: Remove cargo-cult 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.

meson.build

@@ -55,12 +55,8 @@ endforeach
 #
 
 feature_defines = [
-    ['_REENTRANT',                  1], # Define so that reentrant versions of several functions get declared
-    ['_THREAD_SAFE',                1], # Same as _REENTANT for some other OSes
     ['_GNU_SOURCE',                 1], # Enable GNU extensions on systems that have them
-    ['_POSIX_PTHREAD_SEMANTICS',    1], # Enable threading extensions on Solaris
-    ['__EXTENSIONS__',              1], # Enable general extensions on Solaris
-    ['_FILE_OFFSET_BITS',           64], # Define to 64 for large files support
+    ['_POSIX_C_SOURCE',             '200809L'], #IEEE Std 1003.1
 ]
 
 if host_machine.system() == 'windows'
@@ -69,14 +65,6 @@ if host_machine.system() == 'windows'
             ['UNICODE',                     1], # Define to 1 for Unicode (Wide Chars) APIs
             ['_UNICODE',                    1], # Define to 1 for Unicode (Wide Chars) APIs
             ['__USE_MINGW_ANSI_STDIO',      1], # Define to force use of MinGW printf
-            ['_ISOC99_SOURCE',              1], # Extensions to ISO C89 from ISO C99
-            ['_ISOC11_SOURCE',              1], # Extensions to ISO C99 from ISO C11
-            ['_POSIX_SOURCE',               1], # IEEE Std 1003.1
-            ['_POSIX_C_SOURCE',             '200809L'], #IEEE Std 1003.1
-            ['_XOPEN_SOURCE',               700], # POSIX and XPG 7th edition
-            ['_XOPEN_SOURCE_EXTENDED',      1], # XPG things and X/Open Unix extensions
-            ['_BSD_SOURCE',                 1], # ISO C, POSIX, and 4.3BSD things
-            ['_SVID_SOURCE',                1], # ISO C, POSIX, and SVID things
     ]
 endif