v0.4.x should have the ability to use dynamic builds

[Michael Lamparski]

This is a big reason why my own code still uses v0.3.x. I need dynamic linking for the occasional time I need to modify the LAMMPS source.

Back when I was writing 0.4.x, I briefly considered having static and dynamic flags, but chickened out. Kornelski gives some good best practices here: https://kornel.ski/rust-sys-crate

In Cargo.toml you can have [features] section with static and dynamic options.

[features]
static = []
dynamic = []

Don't put any of them as Cargo's default feature, because it's too hard to unset defaults in Cargo.

and this gives the toplevel binary crate the power to choose which to use by depending on lammps-sys and activating the appropriate feature.

[Michael Lamparski]

v0.5.x added this ability back, done using pkg-config with automatic builds as a fallback