macOS: fix broken .pc file for libmtp

This has bugged me forever. The existing file creates a warning on every single
compiler invocation. I really need to figure out if I can get this fixed
upstream. But while I'm at it, I submitted it here to make it easier to spot
warnings in the build output.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

scripts/build.sh

@@ -437,6 +437,7 @@ if [[ $PLATFORM = Darwin && "$BUILD_DEPS" == "1" ]] ; then
 
 	./${SRC_DIR}/scripts/get-dep-lib.sh single . libmtp
 	pushd libmtp
+	patch -p1 < ../${SRC_DIR}/scripts/libmtp.patch
 	echo 'N' | NOCONFIGURE="1" bash ./autogen.sh
 	mkdir -p build
 	cd build

scripts/libmtp.patch

@@ -0,0 +1,13 @@
+diff --git a/libmtp.pc.in b/libmtp.pc.in
+index 93c7bbf..ff817e1 100644
+--- a/libmtp.pc.in
++++ b/libmtp.pc.in
+@@ -10,6 +10,6 @@ Description: libmtp is a library for accessing Media Transfer Protocol devices
+ Version: @VERSION@
+ Requires.private: @LIBUSB_REQUIRES@
+ Conflicts:
+-Libs: -L${libdir} -lmtp
++Libs: -L${libdir} -lmtp @OSFLAGS@
+ Libs.private: @LIBS@
+-Cflags: -I${includedir} @OSFLAGS@
++Cflags: -I${includedir}