Don't build and install the debug library by default

Normally this isn't needed, and there is unfortunately a lot of
hassle getting libraries shared libraries that use template
code working across compilers (right now I'm struggling with
Sun Studio 10 that seems to put local scope on some of the symbols)

Change-Id: Ia35e08211748b039333668613841dd5bc7a03a24
Reviewed-on: http://review.couchbase.org/24276
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>