Mac: Fix compilation with -sdk option

Mac: Fix compilation with -sdk option

The include paths that the mkspecs set into the OpenGL/AGL frameworks
must take the SDK into account.
(The headers in 10.8 have slightly changed wrt 10.7, leading to compile
errors because framework style includes (<OpenGL/foo.h>) within the system
headers are resolved to the SDK framework headers.)

Change-Id: I6113cdb95b462d587f593682e03e81e920f3f672
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>