Linux: build with clang out-of-the-box, without need

Linux: build with clang out-of-the-box, without need
to specify -Dclang=1 gyp define.

This is helpful for Linux distros and portability, and helps
to avoid compiler detection at run time, or explicit configuration
for a specific compiler.

See https://bugs.gentoo.org/show_bug.cgi?id=422829

BUG=http://code.google.com/p/chromium/issues/detail?id=133082
TEST=none

Committed: http://src.chromium.org/viewvc/native_client?view=rev&revision=10067

[Mark Seaborn, 2012-10-22 15:40:01]

LGTM

[commit-bot: I haz the power, 2012-10-22 16:57:39]

CQ is trying da patch. Follow status at
https://nativeclient-status.appspot.com/cq/phajdan.jr@chromium.org/11234020/1

[commit-bot: I haz the power, 2012-10-22 17:03:50]

Try job failure for 11234020-1 (retry) on nacl-lucid32_newlib_opt for steps
"annotate, gyp_compile, summary".
It's a second try, previously, steps "annotate, gyp_compile, summary" failed.
http://build.chromium.org/p/tryserver.nacl/buildstatus?builder=nacl-lucid32_n...

[Paweł Hajdan Jr., 2012-10-22 20:18:42]

Mark, could you take another look? I've hopefully fixed the problem with
redefining compiler symbols here. -fno-builtin seems sufficient compared to
-ffreestanding.

[Mark Seaborn, 2012-10-22 21:04:15]

LGTM

[commit-bot: I haz the power, 2012-10-22 21:04:58]

CQ is trying da patch. Follow status at
https://nativeclient-status.appspot.com/cq/phajdan.jr@chromium.org/11234020/5004

[commit-bot: I haz the power, 2012-10-22 23:13:28]

Change committed as 10067