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.
Committed: http://code.google.com/p/gyp/source/detail?r=1031

[Evan Martin, 2011-09-06 17:27:13]

I hate how complex this stuff (make variables, not your patch) is.  Are you sure
it doesn't make more sense to just require these variables to be set at gyp
time?  (That would also prevents the common for me problem of forgetting to set
CC to the proper value in a new terminal, building, and wondering why it's
rebuilding everything from scratch.0

[Nico, 2011-09-06 19:06:17]

New CL up, ptal.

(Posterity: there was an email exchange where Evan said he'd like to set CC
during gyp time instead; and I, uhm, retorted that some tools expect envvars to
override makefiles, and now I'm answering Evan's plea for simplicity with a 60%
longer patch)

[Evan Martin, 2011-09-06 19:12:57]

LGTM

[Mark Mentovai, 2011-09-07 01:34:27]

LGTM