This change adds missing variables in mkspecs/qconfig.pri about gcc
compiler version when using mingw: QT_GCC_{MAJOR,MINOR,PATCH}_VERSION
This is needed in case CONFIG += c++14 is used.

Task-number: QTBUG-44142
Change-Id: I34c27f9154bb745a8ee75c777a0acbdbc5bda5a9
Reviewed-by: Louai Al-Khanji <louai.al-khanji@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>