Update the buildsystem for KConfig finding Qt

The buildsystem will now only search for Qt 4 and Qt 5. Support for Qt 2
and 3 was dropped in the previous commits (Qt 3 was EOL'ed in 2010 or
so...).

For Qt 5, to be future-proof with the future direction notice appearing
in the 5.5 release, C++11 support is automatically enabled.

Signed-off-by: Thiago Macieira <thiago.macieira@intel.com>
Signed-off-by: Michal Marek <mmarek@suse.com>