Clarify instruction for building with vcpkg

[technic]

Mention VCPKGRS_DYNAMIC=1 environment variable,
and the fact that you need to run cargo clean in order to force rebuild.

[gwenn]

Would you mind testing the following change:
Add this line:

if cfg!(all(feature = "vcpkg", target_env = "msvc")) {
    println!("cargo:rerun-if-env-changed=VCPKGRS_DYNAMIC");
}

here: https://github.com/jgallagher/rusqlite/blob/master/libsqlite3-sys/build.rs#L133

Add then check that you don't need to clean anymore.

[technic]

@gwenn
I can readily test it on windows. How can tell cargo to use local git instead of upstream crates source?

[gwenn]

https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#testing-a-bugfix

[technic]

Awesome, every time I am gladly surprised how rust developers though through all scenarios in advance.
I would check it!

[technic]

@gwenn
Your fix works. Pull request updated, thanks!

[gwenn]

Thanks.