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
Fix error: format not a string literal and no format arguments [-Werror=format-security]
GLib 2.14 is required
Merge pull request #483 from gusnan/master

Fix build error with DEB_BUILD_HARDENING_FORMAT
Don't clobber data, just adjust url endpoint
Merge pull request #485 from RichardHitt/urlgrab

Don't clobber data, just adjust url endpoint
Supposed fix for license issues

I have no clue what to put here so I'll just use what the About dialog
provides (unless specified otherwise in the file)
Cosmetics
Revert "Cosmetics"

This reverts commit a88b3e8aaf78989f5247cbebe117e76b72d97844.
Use C# style license comments
Disable Unix path detection mechanism
Consistent header macros everywhere
Cosmetics
Add missing include
Another missing include..
Last one