Prevent OS X from picking a wrong version of clang from the env

Until now we relied on the PATH to pick the right clang version on
OS X which turned out to be a source of issues on build nodes that
have a lot of old cruft lying around which was picked up by gyp.
Set make_clang_dir so that gyp uses the configured clang version
from Qt and remove the env manipulation from our gyp script.

Change-Id: I4ac679ea56fa874eecaa578aad77b462445c0caa
Reviewed-by: Michael Bruning <michael.bruning@digia.com>