autotools: FreeBSD support

This patch adds FreeBSD and pkg-config support to configure.ac. The pkg-config support is somewhat dependent on https://github.com/strophe/libstrophe/pull/37, but degrades nicely to the original behavior.

Some of the code was re-used from https://github.com/strophe/libstrophe/pull/22.
openssl, libresolv, libxml2/expat ain't used by profanity directly. After switching to pkg-config libstrophe must provide its dependencies via $libstrophe_LIBS.