Only build against prebuilt sysroot when using clang

Its is not currently possible to use gcc with the sysroot
since the sysroot only contains C++ headers for gcc 4.6
and gcc 4.8 is required to build chromium.  Luckily clang
is more flexible and able to detect and work with the 4.6
headers in the sysroot.

BUG=504446

Review URL: https://codereview.chromium.org/1476923005

Cr-Commit-Position: refs/heads/master@{#361905}
2 files changed