mkqt5bld.py: added strict mode, fixes for mingw build and webkit.

- Added strict mode to script, will default to true, if strict_mode=false
given in command line, script will continue running make and make
install even if some module fails to build.
- Some fixed for mingw build.
- Fixed make install for webkit.

Change-Id: I9954c1aa8386dfc440eea35958c27078075466c9
Reviewed-by: Simo Fält <simo.falt@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>