Use the QMAKE_CXXFLAGS_WARN_ON variable for setting -Werror

This would mean we don't pass -Werror when under CONFIG += warn_off.
However, that's not the main goal. The main goal of this change is to
have -Werror appear *after* -Wall -Wextra.

With some compilers, like Clang, this is necessary to have the
-Wno-error=foo options work properly. For example, if the -Wfoo
warning gets enabled by -Wall, Clang will treat it as an error if the
arguments appear in the following order:
   -Werror -Wno-error=foo -Wall

But not if they appear in this order:
   -Wall -Werror -Wno-error=foo

Change-Id: I38c820bffc8277d909391e9bf557db5347836b9c
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
expand tabs and related whitespace fixes in *.{cpp,h,qdoc}

the diff -w for this commit is empty.

Started-by: Thiago Macieira <thiago.macieira@intel.com>
Change-Id: I77bb84e71c63ce75e0709e5b94bee18e3ce6ab9e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>