make: Create containing directories for manpages if not existent

make: Create containing directories for manpages if not existent

When installing to the default DESTDIR /usr/local with non existent
/usr/local/share/man/man8 (or any subpath of it), the installation
of manpages fails with e.g.

  install: cannot create regular file `/usr/local/share/man/man8/netsniff-ng.8.gz': No such file or directory
  make: *** [netsniff-ng_do_install] Error 1

Thus, create any leading, non-existent directories by using the INST
instead of the INSTX command which will call install -d on the path.

Reported-by: James Burnett <James.Burnett@geant.org>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>