Enable the latest versions of GCC, Clang and ICC with -Werror

Tested with GCC 4.9, Clang from XCode 5.1 and ICC 15 beta.

Clang 3.5 (pre-release) cannot compile qtdeclarative yet with -Werror
due to invalid C++ code there that calls member functions on null
pointers.

Change-Id: Ic2845371a1899716985bc0813dfb820fa418e207
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>