Relax compiler specification on Darwin 9 and 10

Relax compiler specification on Darwin 9 and 10

Commit 1992b702f64f561c75cc74d4645bb0e1afeec6bd made it possible to
relax the compiler choice on a per architecture basis. That commit
allowed the user to specify their compiler of choice on all Linux
systems, but hardcoded 'cc' everywhere else.

This patch permits Darwin 9 and 10 users to also override the default
compiler for user space builds.

Reviewed-on: http://gerrit.openafs.org/501
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>