build: multithreaded individual object builds

Signed-off-by: Dylan Turner <dylan.turner@tutanota.com>

args: version bump

Signed-off-by: Dylan Turner <dylan.turner@tutanota.com>

args: version bump

Signed-off-by: Dylan Turner <dylan.turner@tutanota.com>

Makefile: del, uses itself

Signed-off-by: Dylan Turner <dylan.turner@tutanota.com>

build: correct name of tmp file to be cleaned

Signed-off-by: Dylan Turner <dylan.turner@tutanota.com>

build: rebuild if headers change

Signed-off-by: Dylan Turner <dylan.turner@tutanota.com>

Dockerfile: allow overriding def cmd via env var

Signed-off-by: Dylan Turner <dylan.turner@tutanota.com>

Dockerfile: use docker to build Linux compat bin

I use NixOS, so this is for release

Signed-off-by: Dylan Turner <dylan.turner@tutanota.com>