Log Message:
devel/libatomic-links: Use relative symlinks.

This way they work when cross-compiling too.  For example, instead of

/usr/obj/dest/usr/pkg/lib/libatomic/libatomic.so -> /usr/pkg/lib/libatomic.so

we have

/usr/obj/dest/usr/pkg/lib/libatomic/libatomic.so -> ../libatomic.so

so that when the linker is run with

--sysroot=/usr/obj/dest -L/usr/pkg/lib/libatomic -latomic

it will actually find something.

Out of paranoia, add a warning in case the targets aren't under
${PREFIX}/lib as we expect.