contrib: matroska: fix passing flags to to compiler

Given it wasn't use maybe it's not needed anymore with the switch to cmake.

contrib/src/matroska/rules.mak

@@ -21,9 +21,9 @@ libmatroska: libmatroska-$(MATROSKA_VERSION).tar.xz .sum-matroska
 	$(call pkg_static,"libmatroska.pc.in")
 	$(MOVE)
 
-MATROSKA_EXTRA_FLAGS = CXXFLAGS="${CXXFLAGS} -fvisibility=hidden"
+MATROSKA_CXXFLAGS := $(CXXFLAGS) -fvisibility=hidden
 
 .matroska: libmatroska toolchain.cmake
-	cd $< && $(HOSTVARS_PIC) $(CMAKE) -DBUILD_SHARED_LIBS=OFF
+	cd $< && $(HOSTVARS_PIC) CXXFLAGS="$(MATROSKA_CXXFLAGS)" $(CMAKE) -DBUILD_SHARED_LIBS=OFF
 	cd $< && $(MAKE) install
 	touch $@