Disable sysroot option when clang build is disabled

Disable sysroot option when clang build is disabled

Current sysroot image supports gcc <= 4.6. When clang build is disabled
and gcc > 4.6, builds fails as gcc doesn't find the system headers in
sysroot location. So disabling sysroot option when clang is disabled.
Ref: https://code.google.com/p/chromium/issues/detail?id=504446#c17

Disabling the sysroot option as per discussion below:
https://groups.google.com/a/chromium.org/d/msg/chromium-dev/8BILwhBeVCw/TEjLtO2fAAAJ

BUG=504446
TESTS=NONE

R=dpranke,sbc

[RaviKasibhatla, 2015-11-26 11:05:34]

kphanee@chromium.org changed reviewers:
+ kphanee@chromium.org

[RaviKasibhatla, 2015-11-26 11:05:34]

PTAL!

When adding the condition in the same conditions block where we are setting
sysroot% variable, it was not working. So had to add the condition at one level
deeper than where sysroot% is being set.

In order to have the condition at one level deeper, I had to move use_sysroot%
to even deeper level. Not sure whether they is alternate way to this. If yes,
please suggest.

[RaviKasibhatla, 2015-11-26 11:06:10]

kphanee@chromium.org changed reviewers:
+ dpranke@chromium.org, sbc@chromium.org
- dpranke@google.com, sbc@google.com

[Sam Clegg, 2015-11-26 16:36:13]

On 2015/11/26 11:06:10, RaviKasibhatla wrote:

Ooop,  I already have an equivalent CL:
https://codereview.chromium.org/1476923005/.  I should have CC'd you.

[r.kasibhatla, 2015-11-26 17:31:46]

On 2015/11/26 at 16:36:13, sbc wrote:
> On 2015/11/26 11:06:10, RaviKasibhatla wrote:
> 
> Ooop,  I already have an equivalent CL:
https://codereview.chromium.org/1476923005/.  I should have CC'd you.

No problem. Thanks for the quick patch. :)