Decide automatically whether to use GCC or Clang. The default policy is to prefer GCC, but fall back to Clang if we can't find it. This can be influenced by the environment variable `$CROSS_COMPILER'. As part of this, move setting `CC', etc. from `android-cross- config.site' to `common-cross-config.site'.
Support Clang as a `cross-tool' and use it under Android. Add the necessary machinery to use Clang instead. Annoyingly, the Android NDK doesn't name its Clang wrappers with the usual GNU architecture triples, so there's an exciting amount of hacking to be done. What I don't have yet is a principled way to configure which compiler should get chosen for a given build.
etc/common-cross-config.site: Set `ac_tool_prefix' explicitly. Autoconf will set this from the host alias provided on the command line, but this is done before reading the `config.site' file, so we must do this by hand here. Otherwise, `configure' only finds the generic versions of the tools, which will work if `binutils-multiarch' is installed, but not otherwise.