Remove the magic that sets CXX in the toplevel Makefile
Remove the magic that sets CXX in the toplevel Makefile

The logic was supposed to trigger a rebuild if the CXX variable is
changed. However, it failed to track the other compiler related
variables, and, what is worse, conflicted with the clang=1 gyp setting
by forcing CXX to g++

The new logic just relies on tracking GYP defines, and checks whether
the CXX binary - if set - has a target different from the host
architecture.

BUG=none
R=jkummerow@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/807143003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#25973}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh