This fixes recursed builds not having meta mode enabled for them which
disabled dependency and and command change tracking. We only want common
objects marked .NOMETA when recursing, not non-common objects. The
common code expects _PROGS_COMMON_SRCS does not contain the prog source
or else it will be marked .NOMETA.
Add comments explaining the intent and cases being covered.
Fixes: 4ea5e107b1 ("<bsd.progs.mk>: Allow using SRCS for common sources")