On OS/X, use the darwin subdirectories in the environment.  Makes it
easier for those of us with access to the OS/X based QNX tools to use
Qt Creator on OS/X.

Change-Id: Iebc3a89978e6211781404bfc8a4ba95a3523d8b5
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>