Blind attempt to fix PCH

...after 86ba1843 "Introduce
gb_*_set_external_code"

Change-Id: I6ed9652c6157b7cb2196f044f62c606968ef1807

solenv/gbuild/platform/com_GCC_class.mk

@@ -171,6 +171,7 @@ $(call gb_Helper_abbreviate_dirs,\
 		$(4) $(5) \
 		$(gb_COMPILERDEPFLAGS) \
 		$(if $(VISIBILITY),,$(gb_VISIBILITY_FLAGS)) \
+		$(if $(EXTERNAL_CODE),$(gb_CXXFLAGS_Wundef),$(gb_DEFS_INTERNAL)) \
 		$(6) \
 		$(call gb_cxx_dep_generation_options,$(1),$(call gb_PrecompiledHeader_get_dep_target,$(2))) \
 		-c $(patsubst %.cxx,%.hxx,$(3)) \

solenv/gbuild/platform/com_MSC_class.mk

@@ -79,6 +79,7 @@ $(call gb_Helper_abbreviate_dirs,\
 	unset INCLUDE && \
 	$(gb_CXX) \
 		$(4) $(5) -Fd$(PDBFILE) \
+		$(if $(EXTERNAL_CODE),,$(gb_DEFS_INTERNAL)) \
 		$(gb_LTOFLAGS) \
 		$(gb_COMPILERDEPFLAGS) \
 		-I$(dir $(3)) \