[CMake] Cmake Path Please

[J Decker]

THere's (some) error messages that don't include the full path of the
CMakeLists.txt, so I don't know what 'top level' is.

It would be nice if CMakeLists.txt had the full ${CMAKE_SOURCE_DIR} (?)
prepended to it.

This is built as an external project of another top level project, which
itself is built from some other top level cmakelists.txt.

```
[636/1094] Performing configure step for 'sack_vfs_command_portable'
CMake Warning (dev) in CMakeLists.txt:
  No project() command is present.  The top-level CMakeLists.txt file must
  contain a literal, direct call to the project() command.  Add a line of
  code such as

    project(ProjectName)

  near the top of the file, but after cmake_minimum_required().

  CMake is pretending there is a "project(Project)" command on the first
  line.
```
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