Set the default build type to Debug

May be overridden with BUILD_TYPE set to:
  Debug, Release, RelWithDebInfo or MinSizeRel

Additional options may be passed as:
  EXTRA_CMAKE_OPTIONS

Change-Id: I3047cc97f0c398e9a13866f981ef5d4330490a58
Reviewed-on: http://review.couchbase.org/35488
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>