Make it possible to build even if dependency files can't be generated

If the local system doesn't have GNU C or clang, and not even
makedepend, the build will stop because the call of 'makedepend'
fails.  This changes so the build won't stop because of such failure.
The result will be empty .d files, and that's ok.

Reviewed-by: Rich Salz <rsalz@openssl.org>