Disable libgcj on cygwin, mingw32 and beos

[Tom Tromey]

>>>>> "Alexandre" == Alexandre Oliva <aoliva@redhat.com> writes:

Alexandre> Based on last week's discussions about libgcj, I'm
Alexandre> disabling it on the aforementioned targets, release branch
Alexandre> and mainline.

Thanks.

There are cygwin libgcj patches pending.  I'm inclined to leave them
out of gcj 3.0 given how close we are to the release.  (Not to mention
that I, at least, can't look at them in the near future.)

Tom