Rewrote qmake's #include-detection to be more faithful to CPP.

The C preprocessor allows backslash-newline anywhere and allows
comments anywhere it allows space.  Testing wilfully perverse
applications of that revealed qmake's parsing of #include directives
wasn't very robust.  So rework to actually follow the rules and add
those tests.

Change-Id: If5cc7bfb65f9994e9ab9ed216dd1ee7285c63934
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>