contrib/ebml+mkv: compile with hidden symbols

This solves linking issues on iOS and should have no negative impact on further platforms

(cherry picked from commit 6031c9769a0dbd8a4f484027d5856b12ec9cc188)

contrib/src/ebml/rules.mak

@@ -14,10 +14,10 @@ libebml: libebml-$(EBML_VERSION).tar.bz2 .sum-ebml
 	$(MOVE)
 
 # libebml requires exceptions
-EBML_EXTRA_FLAGS = CXXFLAGS="${CXXFLAGS} -fexceptions" \
+EBML_EXTRA_FLAGS = CXXFLAGS="${CXXFLAGS} -fexceptions -fvisibility=hidden" \
 					CPPFLAGS=""
 
 .ebml: libebml
-	cd $< && $(HOSTVARS) ./configure $(HOSTCONF)
+	cd $< && $(HOSTVARS) ./configure $(HOSTCONF) $(EBML_EXTRA_FLAGS)
 	cd $< && $(MAKE) install
 	touch $@

contrib/src/matroska/rules.mak

@@ -16,7 +16,9 @@ libmatroska: libmatroska-$(MATROSKA_VERSION).tar.bz2 .sum-matroska
 	$(UNPACK)
 	$(MOVE)
 
+MATROSKA_EXTRA_FLAGS = CXXFLAGS="${CXXFLAGS} -fvisibility=hidden"
+
 .matroska: libmatroska
-	cd $< && $(HOSTVARS) ./configure $(HOSTCONF)
+	cd $< && $(HOSTVARS) ./configure $(HOSTCONF) $(MATROSKA_EXTRA_FLAGS)
 	cd $< && $(MAKE) install
 	touch $@