Log Message:
databases/postgresql*: Work around autoconf mkdir bug

autoconf does not accept NetBSD mkdir (this bug has already been fixed
in upstream autoconf).  This leads to install.sh, and thus
postgresql's workaround for a relative path in pgxs, and then that
being used in postgis.  Somewhere in that chain there is a bug leading
to postgis failing to make a directory at install time.

This passes in an autoconf cache variable so that for mkdir it uses
mkdir (which it does on GNU/Linux, and thus that situation is
well-tested).