Allow passing in custom RUSTFLAGS

[opoplawski]

Also, according to https://cmake.org/cmake/help/latest/command/add_custom_command.html the ARGS option is ignored.

[micahsnyder]

Most of this makes sense to me, with the string(APPEND RUSTFLAGS " -g") and string(STRIP "${RUSTFLAGS}" RUSTFLAGS).
Removing ARGS also makes sense, after looking at the docs to see that the minimum cmake version we require says it is optional/ignored.

I am a little confused about one part of this change. Why do you want to remove the quotes around ${RUSTFLAGS}? It seems like that might break things if you have more than one flag.

[opoplawski]

The quotes caused problems. It seems like cmake automatically quotes the arguments.

[micahsnyder]

The quotes caused problems. It seems like cmake automatically quotes the arguments.

Very interesting. I didn't expect it to work this way, but I did some manual testing using options -D RUSTFLAGS="-g -v" and it seems good.

We may also consider adding some documentation to the top of this file explaining that you can set RUSTFLAGS cmake variable before using this module. But I don't feel strongly about it. I ran this through our automated tests a couple weeks ago and that looked good. May as well merge now.

Thanks for the PR @opoplawski