asn1: Python 2.1 compatibility
Fix RSA object serialization
Fix RSA object serialization: Python 3 compatibility
Use autoconf to generate compiler options

Hopefully this means we'll break on fewer platforms.

Also, remove some of the extra optimization flags (e.g. -O3
-fomit-frame-pointer), which don't really do much.
OpenBSD: Automatically look for libgmp in /usr/local