Makefile: re-run autogen.sh if JAVA_HOME went away

On Fedora, the weekly OpenJDK security updates put the headers into a
different directory each time because the full version of the package is
encoded in the name of the directory.  Then a incremental build breaks
somewhere in jvmfwk because the headers cannot be found in the old
directory.

So add a hack to detect a non-existent JAVA_HOME and re-run autogen.sh
in the hope that the following incremental build with work.

Change-Id: Iada3cfa3e9a0120d38c6b01484f505ff20770b72
Reviewed-on: https://gerrit.libreoffice.org/27339Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>

Makefile.in

@@ -45,7 +45,9 @@ $(BUILDDIR)/config_host.mk : $(wildcard \
 		$(BUILDDIR)/autogen.input \
 		$(BUILDDIR)/autogen.lastrun \
 		$(BUILDDIR)/autogen.sh \
-		)
+		) \
+		$(shell source $(SRCDIR)/bin/get_config_variables JAVA_HOME && \
+			if test -n "$${JAVA_HOME}" -a ! -d "$${JAVA_HOME}"; then echo force-restart; fi)
 	sh -c $(SRCDIR)/autogen.sh
 
 else # MAKE_RESTARTS