Make link time optimization flags configurable
We want to use -flto=auto in Arch Linux to speed up building, but we
can't hardcode it in buildenv/lto.sh because other downstreams might
have clang < 13.0.0 which did not recognize -flto=auto as equivalent
to -flto=full.
Introducing an LTOFLAGS variable to makepkg.conf seems the way to go.
Signed-off-by:
Allan McRae <allan@archlinux.org>