chrome-sdk: Add support for clang.
chrome-sdk: Add support for clang.

1. Add -isystem include paths to clang.  See discussion on
   crbug.com/86037.
2. Add --make flag, and require it to be set when --clang is set.

BUG=None
TEST=Unit tests, built locally.

Change-Id: I24579f6c510d552099b9145ea5b02cac5ea812ba
Reviewed-on: https://gerrit.chromium.org/gerrit/45272
Commit-Queue: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Reviewed-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Tested-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>

bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite