Document qt5_use_modules as obsolete.

Partial forward-port of cb7f32f5b861fe115fa71f64500a5cbb0b643f1b (Mark
qt4_use_modules and qt4_automoc as obsolete., 2013-07-04) from
cmake.git.

Change-Id: I6eb4d1056208e1bfe67f571b6019129a3a201305
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>