don't replace config.sub in gmp

gmp has its own wrapper version of this file and calls the real one
configfsf.sub. within the gcc build, replacing the wrong one is
harmless, but it breaks independent use of the patched gmp tree and
it's logically wrong.

patch by Laurent Bercot.

Makefile

@@ -136,7 +136,7 @@ musl-git-%:
 	mkdir $@.tmp
 	( cd $@.tmp && $(COWPATCH) -I ../$< )
 	test ! -d patches/$@ || cat patches/$@/* | ( cd $@.tmp && $(COWPATCH) -p1 )
-	test ! -f $</config.sub || ( rm -f $@.tmp/config.sub && cp -f $(SOURCES)/config.sub $@.tmp/ && chmod +x $@.tmp/config.sub )
+	if test -f $</configfsf.sub ; then cs=configfsf.sub ; elif test -f $</config.sub ; then cs=config.sub ; else exit 0 ; fi ; rm -f $@.tmp/$$cs && cp -f $(SOURCES)/config.sub $@.tmp/$$cs && chmod +x $@.tmp/$$cs
 	rm -rf $@
 	mv $@.tmp $@