configure.ac: remove check for mktime

mktime is not used anywhere in the code, and the stock configure
check for mktime invokes UB, so lets just remove it.

closes #71
configure.ac: remove check for mktime

mktime is not used anywhere in the code, and the stock configure
check for mktime invokes UB, so lets just remove it.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>