Rust flags should not be overwritten by default

[xermicus]

Set the value of RUSTFLAGS (set to empty string to avoid overwriting existing flags)

Why do people have to set it to an empty string to not overwrite them? That doesn't follow to me; instead what seems obvious to me would be to not touch the rust flags if rustflags is omitted, and, well, just set them to whatever the value of rustflags is if it was specified.

[athei]

Strongly agree. The default should not override pre-existing flags. For example, it is customary to set architecture specific flags in config.toml that are required to build a project.

[jonasbb]

The topic is already discussed here #48