use $(OS_FOR_BUILD) to fix mingw cross-compilation

Change-Id: I606e99e80ace5b086210e5a724a3e979e2b98cbf

RepositoryExternal.mk

@@ -2898,7 +2898,7 @@ $(call gb_Executable_add_runtime_dependencies,gengal.bin,\
 	$(call gb_Configuration_get_target,registry) \
 	$(call gb_Configuration_get_target,fcfg_langpack) \
 	$(call gb_Library_get_target,$(gb_CPPU_ENV)_uno) \
-	$(if $(filter-out MACOSX WNT,$(OS)), \
+	$(if $(filter-out MACOSX WNT,$(OS_FOR_BUILD)), \
 		$(call gb_Library_get_target,vclplug_svp) \
 	) \
 	$(call gb_Package_get_target_for_build,cppuhelper_unorc) \

solenv/gbuild/Gallery.mk

@@ -59,7 +59,7 @@ $(call gb_Output_announce,$(2),$(true),GAL,1)
 $(call gb_Helper_abbreviate_dirs,\
 	rm -f $(call gb_Gallery_get_workdir,$(2))/* && \
 	$(call gb_Helper_print_on_error,\
-		$(if $(filter-out MACOSX WNT,$(OS)),SAL_USE_VCLPLUGIN=svp) \
+		$(if $(filter-out MACOSX WNT,$(OS_FOR_BUILD)),SAL_USE_VCLPLUGIN=svp) \
 		$(call gb_Executable_get_command,$(gb_GENGAL),$(ICECREAM_RUN)) \
 			$(call gb_Gallery__make_env_args) \
 			--build-tree \

solenv/gbuild/extensions/pre_BuildTools.mk

@@ -8,7 +8,7 @@
 #
 
 
-ifeq (,$(filter WNT,$(OS)))
+ifeq (,$(filter WNT,$(OS_FOR_BUILD)))
 	gb_GENGAL = gengal.bin
 else # windows appends the .exe
 	gb_GENGAL = gengal