it fails because when libc has no REG_STARTEND, some compat code is activated via the NO_REGEX makefile knob, which also adds an include dir with glibc's regex impl and it's supposed to use the regex.h header from there, however, due to use of -isystem in the default xcompile config it uses the system header instead of the one added via -I.
we have to find a way to make MCM use the rootfs's include/ in the exact same way as if it was the compiler's hardcoded default header path.