Make mock wrapper scripts honor 'srcdir'

We're using it in the code already -

Change-Id: I46c1a02b8ed9ab26f5c51592860b8865c8db5927
Reviewed-on: http://review.couchbase.org/28045
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>