filter standard paths from mysql_config & pg_config

filter standard paths from mysql_config & pg_config

standard paths should never be added to compiler/linker lines, as they
are likely to mess up the lookup order.
pkg-config does that filtering for us, but the home-grown config tools
don't, so we need to take care of it.

configure.exe does not have such auto-detection, so the change is not
necessary there.

Task-number: QTBUG-26850
Change-Id: I2f523d5cffb27c3d0a16cdef6ca8a4877c9983c0
Reviewed-by: Mark Brand <mabrand@mabrand.nl>
Reviewed-by: Mitch Curtis <mitch.curtis@nokia.com>
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@nokia.com>