Use GNU AR for illumos

[smklein]

When building some crates that make use of cc-rs in their build scripts, I noticed that cargo build died when running on illumos targets.

Specifically, I saw the following output:

$ cargo build

...

cargo:warning=ar: one of [drqtpmx] must be specified

  exit code: 1

  --- stderr

When supplying the manual environment variable AR=..., I noticed this issue could be avoided. However, the default ar implementation on illumos (in /usr/bin/ar) continued to be incompatible.

This PR updates the default, meaning that cargo build should now "cleanly work" without additional environment variables, by utilizing the "GNU AR" binary bundled with illumos (at /usr/bin/gar) instead of the illlumos-specific one.

Tested this out by compiling ring for illumos - before this patch, it failed, raising the ar issue. After this patch, it builds successfully with no errors.

[smklein]

@jclulow , FYI

[jclulow]

Thanks for the heads up -- I'll take a look!

[alexcrichton]

Thanks!