Modify inclusion to use system icu headers when -Duse_system_icu=1
Modify inclusion to use system icu headers when -Duse_system_icu=1

The gyp definition should take care of the default case
-Duse_system_icu=0 by prepending third_party/icu/public/i18n to the
inclusion search path.

See downstream gentoo bug report:
http://bugs.gentoo.org/show_bug.cgi?id=348944

Contributed by Mike Gilbert <floppymaster@gmail.com>.

BUG=none
TEST=Remove third_party/icu/public/i18n/unicode/usearch.h and build with
-Duse_system_icu=1.

Review URL: http://codereview.chromium.org/6078001

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@69811 0039d316-1c4b-4281-b951-d872f2087c98

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