autotools: refactored configure.ac

This patch implements request #269.
configure.ac: added check for wget_wch support
configure.ac: glib must be at least 2.26

Profanity uses GDateTime structure that appeared in glib-2.26.
configure.ac: removed autoconf version check

autoconf-2.63 works well
configure.ac: added check for expat without pkg-config

and some refactoring
Merge remote-tracking branch 'pasis/iss269'

Conflicts:
	configure.ac