qt: fix postgresql plugin compile

qt: fix postgresql plugin compile

Add qt4 '-psql_config' configure option and use it to set
pg_config path in configure step instead of legacy PSQL_LIBS
environment variable.

Fixes [1]:

PostgreSQL support cannot be enabled due to functionality tests!
 Turn on verbose messaging (-v) to ./configure to see the final report.
 If you believe this message is in error you may use the continue
 switch (-continue) to ./configure to continue.
make: *** [/ssd1/thomas/autobuild/instance-0/output/build/qt-4.8.6/.stamp_configured] Error 101

Patch suggested upstream ([2]).

[1] http://autobuild.buildroot.net/results/1e9/1e949d4593836bc45a55e071cc54ff9c00fad8ae
[2] https://codereview.qt-project.org/108345

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>