Clang looks for the presence of crtbegin.o to decide whether to add GCC standard
C++ library directories to the system header lookup path.

The presence of usr/lib/gcc/x86_64-linux-gnu/4.6/crtbegin.o and
usr/include/c++/4.6 inside this sysroot will cause clang to add the following
directories to the path:
  - $SYSROOT/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../include/c++/4.6
  - $SYSROOT/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../include/x86_64-linux-gnu/c++/4.6
  - $SYSROOT/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../include/x86_64-unknown-linux-gnu/c++/4.6
  - $SYSROOT/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../include/c++/4.6/backward

As well as the standard paths:
  - $SYSROOT/usr/local/include
  - $SYSROOT/include
  - $SYSROOT/usr/include

See clang's lib/Driver/ToolChains.cpp:
  - FilterNonExistent: checks for crtbegin.o
  - Linux::AddClangCXXStdlibIncludeArgs: looks for various c++ directories