The Felix based script to build "flxg" compiler now works.

At least on OSX ..

Requires repository = current directory.
Does not yet build dypgen executable, cheats by copying
from build/release.

The system is a LOT of a mess, it does fairly general
processing of Ocaml mli, ml, mll, mly and dyp files,
however it makes assumptions about the filesystem layout
and names, for example generated *.cmxa libraries
always go in tmp-dir/*.cmxa so they had better not
clash with each other, or with anything else!

Cheats on the version number (uses the one in
host/plat/config.flx). Uses possibly posix specific
time processing functions to stamp the compile version
record. Generates executable "flxg" without suffix
(which is wrong for Windows).

However, with a bit more work this tool together with
flx_build_rtl, reduces the need for Python and fbuild
to constructing a suitable configuation (most of which
is setting the MACROs in the rtl_config.hpp file to
condition the C++ rtl build).