These become fine with C23 because () starts to mean (void) then, but for
previous language versions, it's deprecated, and it causes an annoying
warning when building with Clang by default.
Plus, GCC lacks specific flags to trigger what C23 *does* ban, so a lot
of people are going around building with -Wstrict-prototypes, so let's
just fix this to be consistent with the rest of the codebase anyway
to fend off false positive reports.
- drop old build system
- move shared include and source files to common directory
- drop "rc-" prefix from shared include and source files
- move executable-specific code to individual directories under src
- adjust top-level .gitignore file for new build system
This closes#489.
