Mirtoconf v2: Build.sh Next Generation for mksh and friends

The build system of mksh, nicknamed “mirtoconf”, has grown a lot in the past. It should be revamped, with a few criteria: reusability (jupp, paxmirabilis, possibly mirmake); build/host split (mksh, first for signals, then for Mirkev and MirJSON pregeneration); not rely on CPPFLAGS to signal things to Build.sh (use flags instead); have severities for flags (e.g. MKSH_GCC55009 is a liability; MKSHRC_PATH for experts only, etc); use namespaces for flags; be self-documenting (using grep?); offer writing a config.h as alternative (for RT on Minix 2 and Xenix)