Build testing infrastructure

For the actual testing script, see `sh/test.sh´, but note the following
details :—

• The file `cgirls.h´ has been renamed `aa.h´ because it will be needed
in test binaries as well.

• Source files are now automatically found rather than needing to list
them explicitly in `make/cgirls.mak´.

• Support for a `make/config.mak´ configuration file was added, for
modifying the build. A sample is provided which offers the sort of
optimizations one might want in a production environment.

• Tests consist of comparing result of running a program with the input
in `test/´ against an expected output in `expect/´. Lines which start
with a `#´ are ignored in both. This is to enable REUSE‐conformance
(all of these files are `CC0-1.0´) and hopefully will not cause
problems down the road.