Makefile: try to fix downloading externals for build platform

When cross-compiling, it's possible that an external is only used on
the build platform and not the host platform.  Attempt to fix this by
invoking "make fetch" twice; it looks like this should not be
combined with the exising "build-tools" invocation because that needs to
be delayed until everything is fetched.

Change-Id: I42626a30359d54dce3c0a5a8ea07045c099cea59
Reviewed-on: https://gerrit.libreoffice.org/37912Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>

Makefile.fetch

@@ -96,7 +96,7 @@ endef
 
 download: $(WORKDIR)/download
 
-$(WORKDIR)/download: $(BUILDDIR)/config_host.mk $(SRCDIR)/download.lst $(SRCDIR)/Makefile.fetch
+$(WORKDIR)/download: $(BUILDDIR)/config_$(gb_Side).mk $(SRCDIR)/download.lst $(SRCDIR)/Makefile.fetch
 	-@mkdir -p $(TARFILE_LOCATION)/tmp
 	@date >> $(fetch_LOGFILE)
 	$(foreach item, \

Makefile.in

@@ -275,6 +275,8 @@ help showmodules gbuildtojson:
 	$(MAKE) $(PARALLELISM_OPTION) $(GMAKE_OPTIONS) -f $(SRCDIR)/Makefile.gbuild $@
 
 cross-toolset: bootstrap fetch
+	# fetch again in case there are externals only needed on build platform
+	$(MAKE) gb_Side=build $(PARALLELISM_OPTION) $(GMAKE_OPTIONS) -f $(SRCDIR)/Makefile fetch
 	$(MAKE) gb_Side=build $(PARALLELISM_OPTION) $(GMAKE_OPTIONS) -f $(SRCDIR)/Makefile.gbuild build-tools