support for GNU configure syntax

Re-created: https://github.com/mozilla/rust/pull/9760

Now, without adding `pc-linux-gnu` platform

Re-created: #9565

t will be useful for unix systems, some package managers and external tools that is passing / maybe looking for GNU configure syntax options.

One of related bugs: #5138

Also that could be useful in future if project will really install man pages / other stuff using those variables.