OpenBSD usually use an alternative compiler (`egcc') from ports. But the
`ar' is unprefixed as it comes from base.

the backtrace test doesn't work on openbsd as it doesn't
have support for libbacktrace without using filename.

the diff extends build_helper to provide an function to return the
expected name of GNU-make on the host: "make" or "gmake".

Fixes rust-lang#38429

stdc++ is from base, and is an old library (GCC 4.2)
estdc++ is from ports, and is a recent library (GCC 4.9 currently)

as LLVM requires the newer version, use it if under OpenBSD.