cmake: properly pass host cflags/ldflags to the build

This should hopefully fix build issues where libraries that we ship in
tools/ were accidentally picked up from other places on the system, e.g.
/usr/local

Signed-off-by: Felix Fietkau <nbd@nbd.name>

tools/cmake/Makefile

@@ -24,7 +24,8 @@ HOST_CONFIGURE_CMD := \
 	$(BASH) ./configure \
 		$(if $(MAKE_JOBSERVER),--parallel="$(MAKE_JOBSERVER)")
 
-HOST_CONFIGURE_VARS :=
+HOST_CONFIGURE_VARS += \
+	CXXFLAGS="$(HOST_CFLAGS)"
 
 HOST_CONFIGURE_ARGS := \
 	--prefix=$(STAGING_DIR_HOST)