Let environment variables override make_global_settings.
Let environment variables override make_global_settings.

?= don't work for CC and CXX, as ?= is equivalent to

     ifeq ($(origin FOO), undefined)
       FOO = bar
     endif

and CC's and CXX's origin is "default". See
http://www.gnu.org/software/make/manual/make.html (search for
"conditional variable assignment" and "The origin function").
Without the ifeq and just ?=, CC and CXX wouldn't be overwritten.

With just = instead of ?=, environment variables wouldn't overwrite
CC and CXX, which would break the goma make build instructions.
Review URL: http://codereview.chromium.org/7779041

csulmone/gyp,csulmone/gyp,csulmone/gyp,csulmone/gyp