Fix building libgmp on cygwin

gmp/ghc.mk

@@ -125,6 +125,12 @@ libraries/integer-gmp/gmp/libgmp.a libraries/integer-gmp/gmp/gmp.h:
 	cat $(GMP_TARBALL) | $(BZIP2_CMD) -d | { cd libraries/integer-gmp/gmp && $(TAR_CMD) -xf - ; }
 	mv libraries/integer-gmp/gmp/$(GMP_DIR) libraries/integer-gmp/gmp/gmpbuild
 	chmod +x libraries/integer-gmp/gmp/ln
+
+	# Their cmd invocation only works on msys. On cygwin it starts
+	# a cmd interactive shell. The replacement works in both environments.
+	mv libraries/integer-gmp/gmp/gmpbuild/ltmain.sh libraries/integer-gmp/gmp/gmpbuild/ltmain.sh.orig
+	sed 's#cmd //c echo "\$$1"#cmd /c "echo $$1"#' < libraries/integer-gmp/gmp/gmpbuild/ltmain.sh.orig > libraries/integer-gmp/gmp/gmpbuild/ltmain.sh
+
 	cd libraries/integer-gmp/gmp; (set -o igncr 2>/dev/null) && set -o igncr; export SHELLOPTS; \
 	    PATH=`pwd`:$$PATH; \
 	    export PATH; \