Do not install empty directories; add ASTLIBDIR

Do not install empty directories; add ASTLIBDIR

r368830 modified the installation script to only create a directory if that
directory does not exist.  If some directory variable was empty, it would attempt
to create the empty location.  It also failed to create the ASTLIBDIR directory.
This patch fixes it such that the correct directories are made and only created if
a value specifying them actually exists.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@368852 65c4cc65-6c06-0410-ace0-fbb531ad65f3