ntp: remove empty libexecdir to prevent potential QA issues

Depending on the configuration used to build ntp it is possible to
have an empty libexecdir. This can cause QA issues. Add a test at the
end of install() to remove libexecdir if it is empty, thus avoiding
the possibility of QA issues, regardless of configuration.

Signed-off-by: Mark Asselstine <mark.asselstine@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>