Fix build with precompile headers

Get the CXX compiler using $$QMAKE_CXX instead of ${QMAKE_VAR_QMAKE_CXX}
which causes shell syntax errors when combined with the silent flag.

Task-number: QTBUG-36159

Change-Id: I26cdbe788a20bd2df1aa3563694648e41c082a2c
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>