Add lutro.featureflags table, for use in determining lutro build capabilities

[jstine35]

This is primarily intended for use in internal dev and unit test development. A practical application is that it can be used (soon) to detect whether the lutro build supports the transform rendering feature, and then alter lua graphics test behavior accordingly.

This change was inspired by work done in PR #215 which adds Transform and Scaling features.

I've selected the name lutro.featureflags instead of lutro.config used in PR #215 to improve clarity and avoid ambiguity with various other types of configurations (namely ones coming from data-driven or runtime sources such as ini files, for which the term 'config' is probably more readily associated).

test output

Running the unit tests produces this output:

$ ./runtest.sh test/unit/main.lua
lutro.featureflags.HAVE_COMPOSITION = false
lutro.featureflags.HAVE_JIT = false
lutro.featureflags.HAVE_INOTIFY = false
lutro.featureflags.HAVE_TRANSFORM = false
lutro.featureflags.HAVE_LUASOCKET = false
Lutro unit test run complete