mtools: do not link to libbsd

If libbsd is found by the configuration process, mtools unnecessarily
adds a NEEDED field with libbsd to its dynamic section, but it does not
actually use anything from libbsd under Linux. The same may happen to
host-mtools if some libbsd package is installed on the host machine.

Prevent this by forcing configure to bypass the checking for the
existence of a gethostbyname function in libbsd.

I stumbled on this problem when I built host-mtools and later removed
libbsd to upgrade to Fedora 27, due to Bug 1504831[1]. The previously
built host/bin/mtools started to fail due to the missing libbsd.so.0.

1. https://bugzilla.redhat.com/show_bug.cgi?id=1504831



Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>