Log Message:
acr: update to 2.0.0.

- Add Vinix to the list of supported unix systems
- Add support for SerenityOS
- Add --etcdir as an alias for --sysconfdir
- Use c++ instead of g++ for defaults in CXX
- Add EXEC! keyword. Like EXEC, but with explicit requirement
- Aim to fix the mkdir when no vpath in use
- Add USE_PKGCONFIG directive with support for BSD's pkgconf
- Support pkgconf as an alternative to pkg-config
- Add -qV in the help message
- env_c should not describe CPPFLAGS,
  env_cpp already explains them in more detail.