Configurations/unix-Makefile.tmpl: address find portability issue.

-path is non-portable extension, fortunately it's possible to express
.git subdirectory exclusion with -prune.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7004)