A second try at alexcrichton#163.

The GCC convention is that if the toolchain is named `$target-gnu-gcc`,
then ranlib and ar will be available as `$target-gnu-gcc-ranlib` and
`$target-gnu-gcc-ar` respectively. The code as written would infer them
to be `$target-gnu-{ranlib,ar}`, which will only work if the tools from
`binutils` (which follow that convention) are on `$PATH`.

I've also updated the logic in line with the `cc` crate to check that
the inferred `AR`/`RANLIB` is actually executable before setting it as
an override.

See also rust-lang/cc-rs#736.

Sometimes `gcc-ar` is broken, so when both are available we should
prefer non-gcc-ar. See also rust-lang/cc-rs#741.