OpenGL: Add runtime check for OS X version and improve logic

OpenGL: Add runtime check for OS X version and improve logic

Apple recommend adding NSOpenGLProfileVersion3_2Core when asking for
a 3.2 Core profile context and NSOpenGLProfileVersionLegacy in all other
cases.

Also added a missing runtime check for OS X 10.7 or newer. Fixes a
potential crash if Qt was built on 10.7/8 but executed on 10.6.

Change-Id: I4c09d2dbbe8df25a3553cc01b468dabab0f8eaa4
Reviewed-by: Björn Breitmeyer <bjoern.breitmeyer@kdab.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>