Add note about sysroot not working with icecc

docs/linux_faster_builds.md

Index: docs/linux_faster_builds.md
diff --git a/docs/linux_faster_builds.md b/docs/linux_faster_builds.md
index 625e0cfa951baff92784eb7db946540d73369f6c..dd6285e481bd53f8ab9c822eed174fca7082b8f8 100644
--- a/docs/linux_faster_builds.md
+++ b/docs/linux_faster_builds.md
@@ -40,6 +40,10 @@ When you use Icecc, you need to set some gyp variables.
 
 Icecc doesn't support clang yet.
 
+    use_sysroot=0
+
+Icecc doesn't work with sysroot.
+
 ## Build only specific targets
 
 If you specify just the target(s) you want built, the build will only walk that