configure: Add -D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600 for mingw as well

Mingw headers have got header inline implementations of localtime_r
and gmtime_r, but only visible if certain posix thread safe functions
have been requested.

This is a preparatory step for improving the detection of those
functions.
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -4124,6 +4124,7 @@ probe_libc(){
             add_${pfx}cppflags -D__printf__=__gnu_printf__
         test_${pfx}cpp_condition windows.h "!defined(_WIN32_WINNT) || _WIN32_WINNT < 0x0600" &&
             add_${pfx}cppflags -D_WIN32_WINNT=0x0600
+        add_${pfx}cppflags -D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600
     elif test_${pfx}cpp_condition _mingw.h "defined __MINGW_VERSION"  ||
          test_${pfx}cpp_condition _mingw.h "defined __MINGW32_VERSION"; then
         eval ${pfx}libc_type=mingw32
@@ -4137,6 +4138,7 @@ probe_libc(){
             add_${pfx}cppflags -D_WIN32_WINNT=0x0600
         eval test \$${pfx_no_}cc_type = "gcc" &&
             add_${pfx}cppflags -D__printf__=__gnu_printf__
+        add_${pfx}cppflags -D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600
     elif test_${pfx}cpp_condition crtversion.h "defined _VC_CRT_MAJOR_VERSION"; then
         eval ${pfx}libc_type=msvcrt
         if test_${pfx}cpp_condition crtversion.h "_VC_CRT_MAJOR_VERSION < 14"; then