configure: set blktap2 availability on configure

configure: set blktap2 availability on configure

blktap2 will be enabled by default on Linux systems only, also give
the oportunity to disable it from configure.

Remove the gate in the Makefile that only allows enabling blktap{1,2}
on Linux, since this is now done in a more flexible way on configure.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- reran autogen.sh ]