libvmaf: don't set _XOPEN_SOURCE

This used to set _POSIX_C_SOURCE.  I tried to remove it, because it
caused some functions to be unavailable on BSD, but this broke
Windows, so I settled for setting _XOPEN_SOURCE=600, which exposed the
necessary functions on BSDs.

This no longer works.  libvmaf now uses the non-standard strsep()
function, and FreeBSD only exposes non-standard functions if no
standards macros are defined — they're purely subtractive.  So if
setting _POSIX_C_SOURCE or _XOPEN_SOURCE isn't the right thing to do,
what is?  The build error for Windows is due to a missing M_PI
macro.  mingw provides this macro if any of the standards macros
defined, or _BSD_SOURCE, _GNU_SOURCE, or _USE_MATH_DEFINES.  Since we
already set _GNU_SOURCE for Linux, using that for mingw as well is
probably the best thing to do.  That way, we don't have to set a macro
thta causes FreeBSD to restrict which functions are available.

libvmaf/meson.build

@@ -20,15 +20,12 @@ libvmaf_inc = include_directories(['include'])
 
 # Arguments in test_args will be used even on feature tests
 test_args = []
-if host_machine.system() == 'linux'
+if host_machine.system() == 'linux' or host_machine.system() == 'windows'
     test_args += '-D_GNU_SOURCE'
     add_project_arguments('-D_GNU_SOURCE', language: 'c')
 elif host_machine.system() == 'darwin'
     test_args += '-D_DARWIN_C_SOURCE'
     add_project_arguments('-D_DARWIN_C_SOURCE', language: 'c')
-else
-    test_args += '-D_XOPEN_SOURCE=600'
-    add_project_arguments('-D_XOPEN_SOURCE=600', language: 'c')
 endif
 
 # Header checks