A few fixes on the *nix build system

Hasufell fixed a couple of things in the hexchat build system for the Gentoo ebuild. It seems his github accout is having issues so as a request from him I'm sending the pull request with his changes which (I believe) will benefit hexchat.

The changes are as follow:
* CONFIGURE: fix configure when gtk+ is not installed
* CONFIGURE: don't enable gconf schemas when gtk is disabled
* CONFIGURE: allow to pass specific python version
* MAKE: don't include icons and desktop file for non-gtk builds
* MAKE: install hexchat-plugin.h header if plugins are enabled