Some configurations use "-Os -fomit-frame-pointer" as a default CPPFLAGS. This is incorrect, and at least in the case of system/python3, causes a miscompile on ppc64. This is the true root cause of adelie/packages#451 (closed).
We set it empty in common to ensure that arch-specific overrides can still happen; specifically, if we ever do the once-drafted Pentium 4 spin, we may need some CPPFLAGS for the SSE2 stuff.