gyp: inherit parent for `*.host`

Gyp defaults to gcc/g++ if CC.host/CXX.host is unset. This is not
suitable for environments that only uses the clang toolchain.

Since we already assume that the user will provide clang/clang++
through CC/CXX, lean against it (then drop to gcc/g++).

Also apply the same logic for link/ar for consistency although
it doesn't affect us.

PR-URL: https://github.com/nodejs/node/pull/6173
Fixes: https://github.com/nodejs/node/issues/6152


Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>