Use cargo depfile to prevent calling cargo if not necessary

[jschwe]

• add_custom_command takes an optional DEPFILE command
  • Generator support depends on CMake version
• would allow skipping calling cargo if not necessary
• Blocked on Specify compile target in one crate #201

Concerns:

• What about environment variables used in Rust code (via env! or option_env). Does changing target properties that are used in the custom command via genex properly trigger a rebuild even if a DEPFILE is provided?
• What about include_bytes / include_str - are they tracked in the depfile?

[swilcox3]

@jschwe This feature would be great for me. I'm looking at how to make it so we don't need to call cargo when we do null builds. Right now it looks like the actual build step is a add_custom_target, which runs every time no matter what. I'm tinkering with your source right now to try to see if I can make it a add_custom_command and maybe pass in the generated depfile. You've done fantastic work on this though, and you're definitely more of a CMake wizard than I am, so if you're able to support this officially that'd be great.

[jschwe]

if I can make it a add_custom_command and maybe pass in the generated depfile

I think it should be possible. Some thoughts:

• DEPFILE is not supported by all CMake Generators, although I think the main ones all support it by now.
• The INTERFACE_CORROSION_CARGO_PROFILE target property support might need to be removed to get this feature working. INTERFACE_CORROSION_CARGO_PROFILE currently allows users to set the cargo build profile (release / debug / ...) per target, but is incompatible with specifying the cargo output build artifacts as an OUTPUT of the custom build command. Corrosion currently works around this by having the cargo build custom target not having any BYPRODUCTS and instead specifying the final byproducts in a POST_BUILD command of the custom target.
  I think the POST_BUILD command is also necessary and needs to be seperate from the main build, due to corrosion supporting the OUTPUT_DIRECTORY CMake variables.