contrib: modplug: Link against libc++ on macOS

libstdc++ is not available anymore in modern sdks.

contrib/src/modplug/mac-use-c-stdlib.patch

@@ -0,0 +1,9 @@
+--- libmodplug-0.8.9.0/libmodplug.pc.in.old	2020-04-29 21:39:14.000000000 +0200
++++ libmodplug-0.8.9.0/libmodplug.pc.in	2020-04-29 21:40:06.000000000 +0200
+@@ -8,5 +8,5 @@
+ Version: @VERSION@
+ Requires: 
+ Libs: -L${libdir} -lmodplug 
+-Libs.private: -lstdc++ -lm
++Libs.private: -lc++ -lm
+ Cflags: -I${includedir} -DMODPLUG_STATIC

contrib/src/modplug/rules.mak

@@ -18,6 +18,9 @@ libmodplug: libmodplug-$(MODPLUG_VERSION).tar.gz .sum-modplug
 	$(APPLY) $(SRC)/modplug/modplug-win32-static.patch
 	$(APPLY) $(SRC)/modplug/macosx-do-not-force-min-version.patch
 	$(APPLY) $(SRC)/modplug/fix-endianness-check.diff
+ifdef HAVE_MACOSX
+	$(APPLY) $(SRC)/modplug/mac-use-c-stdlib.patch
+endif
 	$(call pkg_static,"libmodplug.pc.in")
 	$(MOVE)