Fall back to c_rehash if 'openssl rehash' fails

'openssl rehash' isn't implemented on all platforms, and since 'make
test' depends on a rehash of certs/demo being performed, it becomes an
effective block from running tests on any platform but Unix, for the
moment.

It's better to fall back to c_rehash and let the tests perform
everywhere.
Reviewed-by: NMatt Caswell <matt@openssl.org>

Makefile.in

@@ -426,13 +426,14 @@ dclean:
 	@set -e; target=dclean; $(RECURSIVE_BUILD_CMD)
 
 rehash: rehash.time
-rehash.time: certs apps
+rehash.time: certs build_apps
 	@if [ -z "$(CROSS_COMPILE)" ]; then \
 		(OPENSSL="`pwd`/util/opensslwrap.sh"; \
 		[ -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 \
+		|| $(PERL) tools/c_rehash certs/demo) && \
 		touch rehash.time; \
 	else :; fi