Build clang --without-llvmgcc and --without-llvmgxx

Build clang --without-llvmgcc and --without-llvmgxx.

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the system, which will likely be out of date relative to the LLVM being built.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
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=96744

[Mark Mentovai, 2011-08-14 21:44:05]

Since we’ll be using this for production, I’d like to at least be able to run
all of the tests and see a clean state before rolling the llvm/clang version.

[Nico, 2011-08-14 21:54:46]

I always run `make test` in llvm/tools/clang, and don't see errors. Where do you
run `make check`? llvm?

But if it make something work for you that didn't work before, lgtm

[Mark Mentovai, 2011-08-14 22:29:19]

thakis@chromium.org wrote:
> I always run `make test` in llvm/tools/clang, and don't see errors. Where do
> you run `make check`? llvm?

Yes, in llvm.

Running “make check-all” in llvm runs all of the LLVM (equivalent to
“make check” in llvm) and all of the clang (equivalent to “make test”
in llvm/tools/clang) tests.