title: Prune Bazel -I flags when using CROSSTOOL to build ARM
body: <p>I wrote a custom CROSSTOOL file that calls the arm-gcc compiler for my Cortex-M target. I specify my flags, including the sysroot via <code>-isystem</code>.</p>
  
  <p>I see that Bazel augments my invocation of gcc with a bunch of extra <code>-I</code> flags. This seems ok, b/c Bazel is adding paths to files generated during the build, etc.</p>
  
  <p>I see that Bazel is also adding a <code>-isystem</code> that I didn't specify:</p>
  
  <p><code>-isystem external/bazel_tools/tools/cpp/gcc3</code></p>
  
  <p>I can't figure out how to get Bazel to suppress this, since by looking at the only occurrence of "gcc3" <a href="https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/rules/cpp/CppHelper.java#L102" rel="nofollow noreferrer">in the code</a>, it looks like it's trying to pull in a specific STL.</p>
  
  <p>Can I tell Bazel not to add this? It's harmless at best, but possibly insidious.</p>