Needed for classpath. GCJ is also needed but that can be dealt with
separately.

Fix compilation with musl by defining _GNU_SOURCE. What's funny here is
that if __USE_GNU gets replaced, the host build fails. The man page says
_GNU_SOURCE for pthread_getattr_np but glibc violates that statement.

Removed classpath dependency. classpaths must select jamvm, not the other
way around.

Removed target whitelist. Switched to blacklist.

Fixed License information.

Various other cleanups.

Signed-off-by: Rosen Penev <rosenp@gmail.com>