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Without this, configure finds llvm-gcc and llvm-g++ if already installed on
the system, which will likely be out of date relative to the LLVM being built.
This causes "make check" to run additional tests using the system llvm-gcc and
llvm-g++. Some of these tests will fail.

Since we don't build or care about llvm-gcc, it's pointless to run these
tests, and it's scary to run your compiler's test suite and see failures.

BUG=none
TEST=cd third_party/llvm-build; make check-all; make unittests
     for i in tools/clang/unittests/*/Release+Asserts/*Tests; do "${i}"; done
Review URL: http://codereview.chromium.org/7646012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@96744 0039d316-1c4b-4281-b951-d872f2087c98