configure: split checks for libkmod >= 14

configure: split checks for libkmod >= 14

PKG_CHECK_EXISTS won't created a cached variable that later messes with
our PKG_CHECK_MODULES check for an explicit version. Unfortunately,
nesting these checks as the code existed lead to an odd error. Rather,
split the checks apart.

This also improves to the error message when the requisite version
isn't found, and supplies the literal version systemd needs.