Add use_system_icu variable to icu.gyp to allow Linux distros to build with s...

Add use_system_icu variable to icu.gyp to allow Linux distros to build with system ICU.

This will require some changes on the browser side to respect USE_SYSTEM_ICU
pre-processor symbol, but our bundled ICU code is in a separate repo, so landing here first.

TEST=compiles?
BUG=28294
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=52764

[Paweł Hajdan Jr., 2010-07-16 21:38:38]

Please review:

jshin: the idea etc
mark: gyp

[Mark Mentovai - out til August, 2010-07-16 21:48:25]

GYP LG otherwise.

http://codereview.chromium.org/3031006/diff/1/2
File icu.gyp (right):

http://codereview.chromium.org/3031006/diff/1/2#newcode15
icu.gyp:15: # TODO(mark): Looks like this is causing the "public" include
directories
Reflow to fit in 80.

http://codereview.chromium.org/3031006/diff/1/2#newcode396
icu.gyp:396: # TODO(mark): Do we need this as a dependent setting on any
platform?
80

http://codereview.chromium.org/3031006/diff/1/2#newcode415
icu.gyp:415: # Since ICU wants to internally use its own deprecated APIs, don't
80

http://codereview.chromium.org/3031006/diff/1/2#newcode429
icu.gyp:429: 'direct_dependent_settings': {
There are tabs in here. There shouldn’t be.

http://codereview.chromium.org/3031006/diff/1/2#newcode447
icu.gyp:447: 'dependencies': ['system_icu'],
Generally, put dependencies first, because it “flows” better for the reader. Do
this in each of these three targets.

[jungshik at google, 2010-07-16 22:00:00]

Windows will never have ICU. Mac OS X uses it internally, but ICU is not
exposed.  So, we might shuffle / rearrange conditions around a bit... well, it
doesn't matter, much. 

It looks good to me (+ Mark's comments)