Attempting to get windows working with these changes, take 6.

This is a cleanup revision of the (previously mentioned on try builds)
re-architecting o fhow the LD_LIBRARY_PATH setup and extension is
handled in order to accommodate Windows' (1.) use of `$PATH` for that
purpose and (2.) use of spaces in `$PATH` entries (problematic for
make and for interoperation with tools at the shell).