Got windows working with these changes.

This is a cleanup revision of the (previously mentioned on try builds)
re-architecting of how 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).

----

In addition, since the code has been rearchitected to pass the
HOST_RPATH_DIR/TARGET_RPATH_DIR rather than a whole sh
environment-variable setting command, there is no need to for the
convert_path_spec calls in maketest.py, which in fact were put in
place to placate Windows but were now causing the Windows builds to
fail.  Instead we just convert the paths to absolute paths just like
all of the other path arguments.

(also removed a call to `env` in one of the Makefiles accidentally
added while debugging.)