Use rerun-if-env-changed in libsqlite3-sys

[Diggsey]

Build scripts which access environment variables should output lines like:

cargo:rerun-if-env-changed=PATH
cargo:rerun-if-env-changed=SQLITE3_LIB_DIR

To indicate that cargo should rerun the build script if those environment variables change.

Without that, it is confusing for people building crates which depend on this one, because even after they configure their environment, the build will fail until they run cargo clean.

[gwenn]

@Diggsey Would you mind give #344 a try ?

[Boscop]

Is it really necessary to rebuild libsqlite3-sys just because PATH changed?
I think this is the cause of my issue https://github.com/jgallagher/rusqlite/issues/435
Would it be possible to exclude PATH from the set of env vars used with rerun-if-env-changed? :)

[gwenn]

@Boscop The directories that are listed in the PATH environment variable.

[Boscop]

@gwenn
Please don't rebuild when the PATH env var changes. It is still SO annoying every day because it massively increases my build times every time when doing cargo check in Sublime and cargo run in the console (because cargo prepends %HOME%\.cargo\bin to PATH even if it's already in PATH).
It causes a rebuild of diesel, migrations_internals, r2d2-diesel, migrations_macros, diesel_migrations and my workspace's crates that depend on diesel. Every time!
It's especially annoying with cargo watch -x run and makes cargo-watch unuseable..

(Please at least allow opting out of the rebuild via a feature flag, but better not do this at all, because a feature flag would require using a [patch] override.)

It should be sufficient to rebuild when the sqlite-specific env vars change!