diff --git a/external/libwebp/ExternalProject_libwebp.mk b/external/libwebp/ExternalProject_libwebp.mk
index 7a709c1bddcf..4bdf7a32174a 100644
--- a/external/libwebp/ExternalProject_libwebp.mk
+++ b/external/libwebp/ExternalProject_libwebp.mk
@@ -46,7 +46,7 @@ $(call gb_ExternalProject_get_state_target,libwebp,build) :
 			--disable-gif \
 			--disable-wic \
 			$(if $(verbose),--disable-silent-rules,--enable-silent-rules) \
-			CXXFLAGS="$(gb_CXXFLAGS) $(if $(ENABLE_OPTIMIZED),$(gb_COMPILEROPTFLAGS),$(gb_COMPILERNOOPTFLAGS))" \
+			CFLAGS="$(gb_CFLAGS) $(if $(ENABLE_OPTIMIZED),$(gb_COMPILEROPTFLAGS),$(gb_COMPILERNOOPTFLAGS))" \
 			CPPFLAGS="$(CPPFLAGS) $(BOOST_CPPFLAGS)" \
 			$(gb_CONFIGURE_PLATFORMS) \
 		&& $(MAKE) \