Attempt to walk a fine line between current usage (/usr/ports which
does an out-of-tree build without setting MAKESYSPATH) and recently
added requirements (JIRA's building the modules in a non-standard
layout). So, when MAKESYSPATH is defined, trust that it will do the
right thing (to catch the JIRA use case). When it isn't defined,
assume a standard FreeBSD tree and reach over to grab bsd.mkopt.mk (to
fix the /usr/ports use case). Both camps cannot be appeased otherwise,
so we have this kludge until it can be sorted out.