add configure flag to disable tests

The tests require the check package to be installed.  There is a configure
test for this package (which is good), but it's unconditional (which is
bad) as it means you can't even build & install kbd w/out the check
package being installed.

URL: https://bugs.gentoo.org/485116
Reported-by: Arfrever Frehtes Taifersar Arahesis <arfrever.fta@gmail.com>
2 files changed