Fetch xmlsec only if necessary

Change-Id: I4c5fe29c539d0f64c6cfede79a832f68feee1205
Reviewed-on: https://gerrit.libreoffice.org/58629
Tested-by: Jenkins
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>

Makefile.fetch

@@ -161,7 +161,7 @@ $(WORKDIR)/download: $(BUILDDIR)/config_$(gb_Side).mk $(SRCDIR)/download.lst $(S
 		$(call fetch_Optional,LIBPNG,LIBPNG_TARBALL) \
 		$(call fetch_Optional,LIBTOMMATH,LIBTOMMATH_TARBALL) \
 		$(call fetch_Optional,LIBXML2,LIBXML_TARBALL) \
-		XMLSEC_TARBALL \
+		$(call fetch_Optional,XMLSEC,XMLSEC_TARBALL) \
 		$(call fetch_Optional,LIBXSLT,LIBXSLT_TARBALL) \
 		$(call fetch_Optional,LPSOLVE,LPSOLVE_TARBALL) \
 		$(call fetch_Optional,LXML,LXML_TARBALL) \