fstools.mk: Use canonical local/sources/ paths for installer and redoxfs
cargo install --path. The recipes/ symlink chain caused cargo to resolve
path dependencies (../libredox, ../redoxfs, ../syscall) relative to the
recipe directory instead of the canonical source location, breaking
the host build.
bison: Add cross-compilation cache variables for POSIX spawn support.
Gnulib's configure couldn't detect the system spawn.h during cross-compile,
causing generated lib/spawn.h to omit #include_next, leaving
POSIX_SPAWN_* constants undefined.
vasilito
b67697918e