FindGTK2: Refactor _GTK2_ADJUST_LIB_VARS into _GTK2_ADD_TARGET

This function avoids creating the targets when the required
dependencies were not found.
Also fix some wrong dependency and some typo.

${FREETYPE_INCLUDE_DIR_ft2build} ${FREETYPE_INCLUDE_DIR_freetype2} are
now required for gtkmm component
FindGTK2: Do not add tests if the relative targets do not exist
FindGTK2: pangocairo test should link to libm (for cos() method)
Merge topic 'FindGTK2-targets' into next

27d4a6a FindGTK2: pangocairo test should link to libm (for cos() method)
e8014c6 FindGTK2: Do not add tests if the relative targets do not exist
b64a821 FindGTK2: Refactor _GTK2_ADJUST_LIB_VARS into _GTK2_ADD_TARGET