(dist-hook): New target, to ensure that config/install-sh

is executable.  Otherwise, on systems that lack a suitable install
binary, `make install' would fail, because of the way this script
is invoked (without `$SHELL ' prefix).

Makefile.am

@@ -23,6 +23,9 @@ ACLOCAL_AMFLAGS = -I m4
 check-root:
 	cd tests && $(MAKE) $@
 
+dist-hook:
+	test -x config/install-sh
+
 distcheck-hook:
 	$(MAKE) my-distcheck