mkqt5bld: Fix system specific directories from qconfig.pri

QMAKE_DEFAULT_LIBDIRS and QMAKE_DEFAULT_INCDIRS don't always
start in the beginning of the line, so let's now find it
in any place of the line and remove everything after the '=' sign.

Task-number: QTBUG-31496
Change-Id: Ie270df673e6fb964688090cbc0ee7cb2b0cc196c
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>