This commit makes the code (mostly) C99-compatible, enabling to compile
it without the -Wno-error=implicit-function-declaration flag. This
way, Magic becomes usable on arm64 architectures, specifically on Apple
computers with M1/M2 SoC.
enabling the "plow" command. At this time, however, not all
rules are being implemented correctly given the numerous changes
to the DRC handling since the last time "plow" was working. Fixes
are ongoing.