This decreases external dependencies for crosscompilation. Note that
these libraries are not built nor used by Minix itself.
Furthermore, the shell scripts that download the tarballs for these
libraries, gcc, binutils, and gmake now also support curl in addition
to wget.
This commit finalizes support for cross compilation. The tools
directory are all links to the actual tools and are built on the
host system to build Minix. build.sh is the work horse that takes
care of all environment settings. It's slightly adjusted for Minix.
The /usr/src/Makefile has additional targets needed for cross
compilation.