Thank you @mitu
That worked, I needed to throw in a --disable-x11 too:
CFLAGS="-O2 -march=armv8-a+crc -mtune=cortex-a72 -mfpu=neon-fp-armv8 -mfloat-abi=hard -ftree-vectorize -funsafe-math-optimizations" ./configure --prefix=/opt/retropie/emulators/retroarch --disable-sdl --enable-sdl2 --disable-oss --disable-al --disable-jack --disable-qt --disable-pulse --enable-opengles --disable-videocore --disable-vulkan --disable-wayland --enable-kms --enable-egl --enable-floathard --enable-neon --disable-x11I may have got rid of the --prefix bit, I can't quite remember now.
Appreciate the help.
Mr Lightgun