rustbuild should permit configuring the "make" program at configure time

[semarie]

On BSD system the "make" program isn't GNU-make, and the Makefile format aren't strictly compatible.

For example, src/libstd/build.rs try to invoke make to build libbacktrace. Same problem in src/bootstrap/check.rs.

The "make" program should be configurable. GNU-make is generally "gmake" on BSD.

[semarie]

I started to work on it.