Fix build break due to rehash command

The build was breaking due to a Makefile recipe expecting an openssl
version to be on the PATH with support for the rehash command.
Reviewed-by: NBen Laurie <ben@openssl.org>

Makefile.org

@@ -428,7 +428,7 @@ rehash.time: certs apps
 		[ -x "apps/openssl.exe" ] && OPENSSL="apps/openssl.exe" || :; \
 		OPENSSL_DEBUG_MEMORY=on; OPENSSL_CONF=/dev/null ; \
 		export OPENSSL OPENSSL_DEBUG_MEMORY OPENSSL_CONF; \
-		openssl rehash certs/demo) && \
+		$$OPENSSL rehash certs/demo) && \
 		touch rehash.time; \
 	else :; fi