Need to explicitly request using libpcre

The Git configure stage apparently requires explicitly requesting using
libpcre for Perl-style regular expressions.  No reason not to have them,
so pass the option that compiles them in.
Add random_seed and trustdb.gpg to diff excludes

The binary files `random_seed` and `trustdb.gpg` are seemingly generated
by the build process and then included in the diff generated for the
source code bundle.  That breaks out-of-the-box compilation of the
source code, so take Marco Atzeri's fix[0] to avoid that.

[0]: https://cygwin.com/ml/cygwin-apps/2014-08/msg00077.html