better not use introduce global variables for the libxml include path

sw/Library_sw.mk

@@ -33,12 +33,6 @@ $(eval $(call gb_Library_add_precompiled_header,sw,$(SRCDIR)/sw/inc/pch/precompi
 
 $(eval $(call gb_Library_set_componentfile,sw,sw/util/sw))
 
-ifeq ($(strip $(SYSTEM_LIBXML)),YES)
-_XML_INCLUDE := $$(LIBXML_CFLAGS)
-else
-_XML_INCLUDE :=
-endif
-
 $(eval $(call gb_Library_set_include,sw,\
     -I$(SRCDIR)/sw/source/core/inc \
     -I$(SRCDIR)/sw/source/filter/inc \
@@ -48,7 +42,7 @@ $(eval $(call gb_Library_set_include,sw,\
     -I$(WORKDIR)/SdiTarget/sw/sdi \
     -I$(WORKDIR)/Misc/sw/ \
     $$(INCLUDE) \
-    $(_XML_INCLUDE) \
+    $(if $(filter YES,$(SYSTEM_LIBXML),$(LIBXML_CFLAGS)) \
     -I$(OUTDIR)/inc/offuh \
     -I$(OUTDIR)/inc \
 ))