libgmp.so is needed for miniruby, that is compiled in host.
Using PKG_BUILD_DEPENDS=gmp/host eventually compiled gmp/host
before ruby/host. However, when this did not happened, build
failed. Using HOST_BUILD_DEPENDS=gmp/host seems to be the
correct way to require it.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>

This commit imports the old Makefile from old openwrt feeds.
Also, the package was updated to the latest version.

The new version was tested in a VM with no aparent problem.
No patches are needed.

I added myself as maintainer. If possible, I would like to gain
commit access.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>