cmake/configure: Allow to specify C/C++ compilers

Change-Id: I965f84e4e0f1528323be9917c2480b86ea61d542
Reviewed-on: http://review.couchbase.org/34566
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>