Makefile: fix fetching for the build platform

The invocation of make fetch added in commit
fe3fb5ec didn't do anything because
the force-restart didn't actually restart make because it touched
"Makefile" but the make read "$(BUILDDIR)/Makefile", and make isn't
smart enough to notice those are the same.

Change-Id: I85cbcba3bf9c3c7b89d4ee33f772820d19542f4a
Reviewed-on: https://gerrit.libreoffice.org/40482Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>

Makefile.in

@@ -30,8 +30,11 @@ GIT_BUILD := $(if $(wildcard $(SRCDIR)/.git),T)
 #    need updated configuration
 ifeq (,$(MAKE_RESTARTS)$(if $(GIT_BUILD),,T)$(if $(filter-out help showmodules clean distclean,$(MAKECMDGOALS)),,T))
 
+# note: this must touch both Makefile and $(BUILDDIR)/Makefile, because make
+# may be invoked using either of these paths, and it will restart itself only
+# if the updated target is exactly the same path as the Makefile it is using
 .PHONY : force-restart
-Makefile: $(BUILDDIR)/config_host.mk $(BUILDDIR)/config_host_lang.mk force-restart
+Makefile $(BUILDDIR)/Makefile: $(BUILDDIR)/config_host.mk $(BUILDDIR)/config_host_lang.mk force-restart
 	@touch $@
 
 # run configure in an environment not polluted by config_host.mk