QColor compilation failure with gcc 4.9

Since commit de82d239f814cf2a717bea0defeee3732384e271 (Make QColor a literal type), QtGui compilation fails with the following message when using gcc 4.9:

src/gui/painting/qcolor.h: In constructor 'constexpr QColor::QColor(int, int, int, int)':
src/gui/painting/qcolor.h:79:18: sorry, unimplemented: use of the value of the object being constructed in a constant expression

Since gcc 4.9 is still supported in Qt 5.14, please fix the issue. This is a known gcc issue that was fixed in gcc 5 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57694).