cmake build system: figure out how to make the zig0 build always execute

[andrewrk]

Currently the cmake build system will not rebuild libuserland.a unless a select number of files are modified:

zig/CMakeLists.txt

Lines 6720 to 6732 in f9ada1c

	add_custom_command(
	OUTPUT "${LIBUSERLAND}"
	COMMAND zig0 ARGS build
	--override-std-dir std
	--override-lib-dir "${CMAKE_SOURCE_DIR}"
	libuserland
	"-Doutput-dir=${CMAKE_BINARY_DIR}"
	WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"
	DEPENDS
	"${CMAKE_SOURCE_DIR}/src-self-hosted/stage1.zig"
	"${CMAKE_SOURCE_DIR}/src-self-hosted/translate_c.zig"
	"${CMAKE_SOURCE_DIR}/build.zig"
	)

I was unable to figure out how to make cmake unconditionally run this zig build command (which does its own caching).

Maybe the #cmake IRC channel could help. I didn't get an easy answer when I asked:

<andrewrk> is there a way to tell add_custom_command to always, unconditionally run the command? the command does its own caching
<andrewrk> can I add something to the DEPENDS that is always considered out of date?
<ngladitz> use a custom target instead?
<andrewrk> I think I tried that and it didn't work for some reason. let me refresh my memory
<andrewrk> ngladitz, because I need to depend on the OUTPUT in a target_link_libraries
<andrewrk> https://github.com/ziglang/zig/blob/f9ada1cfba5140ef01baa138813241c6b93a1ea9/CMakeLists.txt#L6720-L6732
<andrewrk> I'm pretty sure I tried add_custom_target first
<ngladitz> add a target interdependency (add_dependencies()) and declare the output in BYPRODUCTS?
<andrewrk> BYPRODUCTS? are we looking at the same documentation? 
<andrewrk> https://cmake.org/cmake/help/v3.0/command/add_custom_target.html
<andrewrk> actually my cmake min version is 2.8.5
<ngladitz> hm you might start questions with that bit of info .. 2.8.5 is like 8(?) years old
<ngladitz> fwiw BYPRODUCTS was introduces in 3.2 but is only required for Ninja
<ngladitz> *introduced