[PATCH] Fix toolchain detection on SL6

[Rafael Espíndola]

Since we don't seem to need to know the difference from
ScientificLinux6 to ScientificLinux5, I would suggest starting with
just ScientificLinux.

Can you add a test? You should be able to create tree with a
redhat-realease file and, for example, test that we pass
--no-add-needed to the linker (assuming that is correct behavior for
SL6).

Thanks,
Rafael