fcm make: multiple config files and search paths

Can now specify multiple `-f PATH` options for multiple configuration files.

Can now specify multiple `-F PATH` options for searching configuration files
specified as relative paths.

New `include-path` configuration declaration for specifying the search path for
configuration files specified as relative paths.

Improve CLI argument diagnostics.
* Die if an argument is missing an equal sign.
* Suggest command line syntax if argument ends with `.cfg`.

Close #131. Close #132.
This is worth moving into test_header and accepting another argument for the executable's path