CONFIGURE: fix configure when gtk+ is not installed

AM_PATH_GTK_2_0 would be an undefined macro then
CONFIGURE: don't enable gconf schemas when gtk is disabled

This is nitpicky, since we could assume the user wants gconf schema if
gconftool-2 is available. However this can be a wrong assumption.
CONFIGURE: allow to pass specific python version

This is useful if distros support installing multiple python versions
in parallel such as: 2.5, 2.6, 2.7.
For python3.* we will get syntax error during the imports, so the checks
will successfully disable it.
It defaults to python2.
MAKE: don't include icons and desktop file for non-gtk builds
MAKE: install hexchat-plugin.h header if plugins are enabled