Improve Q_CONSTRUCTOR_FUNCTION and Q_DESTRUCTOR_FUNCTION macros.

Improve Q_CONSTRUCTOR_FUNCTION and Q_DESTRUCTOR_FUNCTION macros.

By adding anonymous namespace and static linkage we are reducing
visibility of implementation of these macros.

This patch also fixes warning about a declared but unused variable which
was issued by gcc 4.6 for Q_CONSTRUCTOR_FUNCTION.

Change-Id: I2cb70ad4c93f6f77e5518420abcce6fd4cadccfa
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>