coroutine: adding configure choose mechanism for coroutine backend

Configure tries, as a default, ucontext functions for the
coroutines. But now the user can force another backend by
--with-coroutine=BACKEND option

v2: Using --with-coroutine=BACKEND instead of enable
disable individual configure options

Signed-off-by: Alex Barcelo <abarcelo@ac.upc.edu>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>