qmake: teach moc-detector to handle C++-11 raw strings

As for the #include-parser, the moc-detector's minimal C preprocessor
could be confused by a raw string into ignoring large chunks of code.

Change-Id: Id688e9a1f04628ce75a51a7d15269078c734288e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>