contrib: detect mingw-w64 (as opposed to mingw.org)

(cherry picked from commit 521f40906c11615b9da6fabb19870724b91769fb)
Signed-off-by: Rafaël Carré <funman@videolan.org>

contrib/src/main.mak

@@ -148,6 +148,12 @@ endif
 
 endif
 
+ifdef HAVE_WIN32
+ifneq ($(shell $(CC) $(CFLAGS) -E -dM -include _mingw.h - < /dev/null | grep -E __MINGW64_VERSION_MAJOR),)
+HAVE_MINGW_W64 := 1
+endif
+endif
+
 cppcheck = $(shell $(CC) $(CFLAGS) -E -dM - < /dev/null | grep -E $(1))
 
 EXTRA_CFLAGS += -I$(PREFIX)/include