build: Remake config-host.mak when VERSION changes

configure gets the version number from VERSION, and writes it to
config-host.mak.  The make dependency for that is missing.  Because of
that, a rebuild after a VERSION change may not pick up the change.
Fix that.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <20181214084754.23854-1-armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

Makefile

@@ -67,7 +67,7 @@ CONFIG_ALL=y
 -include config-all-devices.mak
 -include config-all-disas.mak
 
-config-host.mak: $(SRC_PATH)/configure $(SRC_PATH)/pc-bios
+config-host.mak: $(SRC_PATH)/configure $(SRC_PATH)/pc-bios $(SRC_PATH)/VERSION
 	@echo $@ is out-of-date, running configure
 	@# TODO: The next lines include code which supports a smooth
 	@# transition from old configurations without config.status.