FindGTK2: Export GTK2_XXX_FOUND to parent scope in _GTK2_FIND_LIBRARY

Also add a debug message to report if the library was found or not
FindGTK2: Populate GTK2_DEFINITIONS before searching for libraries
FindGTK2: Search for glib-object.h instead of gobject/gobject.h

This is the topmost include file for GObject header files
FindGTK2: Create targets for each library

Methods:
 * _GTK2_ADJUST_LIB_VARS
 * _GTK2_ADD_TARGET_DEPENDS_INTERNAL
 * _GTK2_ADD_TARGET_DEPENDS
are strongly inspired by FindQt4.cmake
FindGTK2: Better handling of include directories

Add a method _GTK2_ADD_TARGET_INCLUDE_DIRS to handle it.
FindGTK2: Do not require the GTK_ prefix in all the internal functions

This saves from using string(REGEXP) to create targets
FindGTK2: Small cleanup
FindGTK2: Add unit tests for gtk and gtkmm targets
Merge topic 'FindGTK2-targets' into next

893c5c1 FindGTK2: Add unit tests for gtk and gtkmm targets
e3003f6 FindGTK2: Small cleanup
8442a39 FindGTK2: Do not require the GTK_ prefix in all the internal functions
7c3adb0 FindGTK2: Better handling of include directories
ab5f03d FindGTK2: Create targets for each library
eeecbe6 FindGTK2: Search for glib-object.h instead of gobject/gobject.h
ed14da3 FindGTK2: Populate GTK2_DEFINITIONS before searching for libraries
fc3b60d FindGTK2: Export GTK2_XXX_FOUND to parent scope in _GTK2_FIND_LIBRARY