Move __cpp_constexpr check inside Q_COMPILER_CONSTEXPR check

Q_COMPILER_CONSTEXPR can be undefined (or not defined at all) to
indicate that constexpr should not be used regardless of the compiler's
ability to support it.  This is done for QNX because some C library
floating point functions used in the Dinkumware C++ library aren't
constexpr functions; i.e., the library doesn't have proper constexpr
support even though the compiler does.

(cherry picked from commit d8724296

)
Change-Id: If0bdeb2180710dd9ccd97d79fa91cf9ff42f7990
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>