Explicitly prefer -ar to -gcc-ar.

[Alexhuszagh]

Fixes #1100.

[Alexhuszagh]

bors try --target -none- --target *-freebsd --target *-dragonfly --target *-solaris --target *-illumos *-netbsd

[Alexhuszagh]

Note I didn't provide AR_$target for the Android images, since these are provided in #1023.

[bors]

try

Build succeeded:

• conclusion

[Emilgardis]

this change is parallel to rust-lang/cc-rs#741, but this is fine standalone and resolves issues for people locked to cc="=1.0.74"

bors r+

[bors]

Build succeeded:

• conclusion

[svenstaro]

Can we get a release containing this? Otherwise this is currently broken by default.

[Alexhuszagh]

I don't think 0.3.0 is quite ready for release, especially not in the next week or so, but this is a trivial patch so we could backport it and do a quick release for 0.2.5?

[Emilgardis]

cc 1.0.75 has been released. rust-lang/cc-rs#741 (comment)

hopefully this solves the problem enough