Configure: count basenames for all library sources

Make sure that each basename only appears once.  This is due to the
static library archiver on Unix, that indexes archived object files by
base name only, thereby making base name clashes...  interesting.

This is a safety net for OpenSSL developer!

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9133)