Also add a basic test for constexpr expressions involving QSizePolicy.
GCC < 4.8.0 has problems with initializing variant members in constexpr ctors:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54922
Instead of revoking its Q_COMPILER_CONSTEXPR, which is a source-incompatible
change, simply note the fact so we can selectively provide constexpr for
QSizePolicy.
Change-Id: Iba8e5a7cdf847d73e8e2b6bb6211fb3c9846aa0a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>