contrib: check is dxva2api.h already exists before install

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit 64aa5fae)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/directx/rules.mak

@@ -20,5 +20,5 @@ $(TARBALLS)/dxva2api.h:
 .directx: directx-oss.tar.bz2 dxva2api.h .sum-directx
 	mkdir -p -- "$(PREFIX)/include"
 	tar xvjf $< -C "$(PREFIX)/include"
-	cp $(TARBALLS)/dxva2api.h "$(PREFIX)/include/"
+	$(CC) -E -include dxva2api.h - < /dev/null >& /dev/null || cp $(TARBALLS)/dxva2api.h "$(PREFIX)/include/"
 	touch $@