Auto merge of #5394 - henriklaxhuber:master, r=alexcrichton

Auto merge of #5394 - henriklaxhuber:master, r=alexcrichton

Pass linker path to build script

This change adds the environment variable LINKER to pass the path of the linker used by cargo to the build script. This complements the variable RUSTC (the rustc binary used) to give the build script full knowledge of its environment.

A specific usage example would be automatically generating bindings to system headers in cross compilation, e.g. by locating jni.h for android targets.