When CONFIG_EXTRA_COMPAT is enabled, grep fails to compile against musl with a whole slew of errors, but works fine with glibc. Not a hugely major issue, as I don't _really_ need EXTRA_COMPAT, but it would be nice if busybox compiled against POSIX-compliant libc with any set of options. Reproducible with `make defconfig; sed -i '/^# CONFIG_EXTRA_COMPAT/cCONFIG_EXTRA_COMPAT=y' .config; make` on a musl-based system (I'm testing in an Alpine chroot).