configure: don't define _POSIX_C_SOURCE for mingw-w64

It forces __USE_MINGW_ANSI_STDIO, M_PI and some functions we already have
done better in compat.

config.h.meson

@@ -41,9 +41,6 @@
 /* Define for Windows 7 APIs. */
 #mesondefine _WIN32_WINNT
 
-/* IEEE Std 1003.1. */
-#mesondefine _POSIX_C_SOURCE
-
 /* Define to 1 to force use of MinGW provided C99 *printf over msvcrt */
 #mesondefine __USE_MINGW_ANSI_STDIO
 

configure.ac

@@ -293,7 +293,6 @@ int foo() { return my_array[0]; }
     ])
     AC_DEFINE([WIN32_LEAN_AND_MEAN],, [Define to limit the scope of <windows.h>.])
     AC_DEFINE([UNICODE], [1], [Define to 1 for Unicode (Wide Chars) APIs.])
-    AC_DEFINE([_POSIX_C_SOURCE], [200809L], [IEEE Std 1003.1.])
 
     AC_ARG_WITH(packagedir,
       AS_HELP_STRING([--with-packagedir=DIR],

meson.build

@@ -347,9 +347,7 @@ windows_version_test = '''
 
         # Defines needed for MinGW
 
-        mingw_defines = [
-            ['_POSIX_C_SOURCE',             '200809L'], #IEEE Std 1003.1
-        ]
+        mingw_defines = []
 
 ucrt_version_test = '''
 #include <crtdefs.h>