qt5base: fix postgresql plugin compile

qt5base: fix postgresql plugin compile

Add qt5base '-psql_config' configure option and use it to set
pg_config path in configure step instead of legacy PSQL_LIBS
enviornment variable.

Fixes [1]:

PostgreSQL auto-detection... ()
make[1]: Entering directory `/home/peko/autobuild/instance-0/output/build/qt5base-5.4.0/config.tests/unix/psql'
/home/peko/autobuild/instance-0/output/host/usr/bin/sh-linux-gnu-g++ -c -pipe -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os --sysroot=/home/peko/autobuild/instance-0/output/host/usr/sh4a-buildroot-linux-gnu/sysroot -O2 -O3 -Wall -W -fPIE  -I../../../mkspecs/devices/linux-buildroot-g++ -I. -I/usr/include/postgresql -o psql.o psql.cpp
sh-linux-gnu-g++: ERROR: unsafe header/library path used in cross-compilation: '/usr/include/postgresql'
make[1]: *** [psql.o] Error 1
make[1]: Leaving directory `/home/peko/autobuild/instance-0/output/build/qt5base-5.4.0/config.tests/unix/psql'
PostgreSQL disabled.
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: *** [/home/peko/autobuild/instance-0/output/build/qt5base-5.4.0/.stamp_configured] Error 101
make: Leaving directory `/home/peko/autobuild/instance-0/buildroot'

[1] http://autobuild.buildroot.net/results/354/354bae2337703ad8bfb9d33c79538df3017b7fe9/

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>