configure: Fix building of rust for release

Set rustver correctly for both release version string
(for example 3.1.7) and git one (3.1.7.1-982f).

corosyncrustver must be escaped by '[]' because sed is using these two
characters and m4 would remove them.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Fabio M. Di Nitto <fdinitto@redhat.com>