Use pkg-config to find Qt 4 and 5 instead of direct qmake

The Qt Project recommendation is that there should always be a "qmake"
binary and it should never be renamed. If it's necessary to handle
multiple Qt versions, the Qt Project recommends using qtchooser.

Unfortunately, some distros do not follow the recommendation, so we
would need to check qmake-qt4, qmake-qt5, etc. So, instead, let's try
pkg-config.

Signed-off-by: Thiago Macieira <thiago.macieira@intel.com>
Reported-by: Alexey Brodkin <abrodkin@synopsys.com>
Acked-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Michal Marek <mmarek@suse.com>