Make configure die if libxml2-dev or equivalent isn't installed.
This probably is wrong since it looks like we're picking libxml2
up from Pidgin, so if it is wrong, feel free to disapprove this rev.
#
# pidgin_major_ver and pidgin_minor_ver should be defined to match the minimum
# Pidgin API version _required_ to build Guifications
# Due to the way Pidgin checks plugin versions, we need to also ensure that
# the correct minimum version of Pidgin is Require:'d based on what version of
# the Pidgin headers we actually build with.
#
# This allows --define 'pidginver 1' to be used to build an rpm for Pidgin 1.x