Ensure -no-pch is effective

Have configure add a "CONFIG -= precompile_header" to qmodule.pri when
-no-pch is specified.  Ensures that Qt is built without precompiled
headers (as requested) even if allowing precompiled header use is the
default for the toolchain.

Parallels changes to Windows configure.

Task-number: QTBUG-11545
Change-Id: Iab4021e74c4e9978770e917dff97b976c449dd8b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>