force gbuild stage for CustomTargets

gb_SourceEnvAndRecurse_STAGE is unset in CustomTarget's make process
when the parent make has been started in sourced environment. This leads
to all sorts of "interesting" failures because the gbuild classes and
platform stuff are not loaded...

solenv/gbuild/CustomTarget.mk

@@ -31,6 +31,7 @@ define gb_CustomTarget__command
 +$(call gb_Helper_abbreviate_dirs,\
 	mkdir -p $(call gb_CustomTarget_get_workdir,$(2)) && \
 	O='$(OUTDIR)' R='$(REPODIR)' S='$(SRCDIR)' W='$(WORKDIR)' gb_AWK='$(gb_AWK)' \
+	gb_SourceEnvAndRecurse_STAGE=gbuild \
 	gb_XSLTPROC='$(gb_XSLTPROC)' GBUILDDIR='$(GBUILDDIR)' SRCDIR='$(SRCDIR)' \
 	$(MAKE) -C $(call gb_CustomTarget_get_workdir,$(2)) -f $< && \
 	touch $(1))