* Add crosstool example; does not work with cmake rule yet
the code is taken from Bazel test data (bazel_toolchain_test_data)
* Make crosstool compilation of cmake_external + cc_binary work
To build example, cd examples/cmake_crosstool bazel build //:libhello_test
Changes:
1) Initially suggested in #124: put $EXT_BUILD_ROOT on path, so that relative paths can be resolved by CMake
2) Toolchain tools are references as relative paths, and they themselves refer to external repository with external/ prefix.
This will not work with CMake, as CMake also perform compiler tests and the build is performed in some temp directory.
We need to convert to absolute paths.
I did a trick with checking of $EXT_BUILD_ROOT is defined and then using it as a prefix.
3) I had to change the visibility of the cc-compiler-k8 toolchain to public
4) For CMake crosstool file, CMAKE_C_COMPILER, CMAKE_CXX_COMPILER, CMAKE_AR need to be absolute, so in cases when they are relative and not under external directory, force paths conversion
