Fix build for FreeBSD

* use /usr/local directory as default prefix for FreeBSD
* include CFLAGS, LDFLAGS, CPPFLAGS to AM_* equivalents
* check that dtrace command is actually working, not just present in
  the system

Change-Id: Ic12e70fe1417ec2c3c7fc4d86e30ff83aab640ee
Reviewed-on: http://review.couchbase.org/25289
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Aaron Hurt <ahurt@anbcs.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>