- Unwrap env variables only when required.
- Handle cases where `CARGO_CFG_TARGET_FAMILY` is not set. There is no
  need to unwrap this env var since we are only using it to optionally
  set some compiler flags.
- For wasm builds, define `ONIG_DISABLE_DIRECT_THREADING`. This is a new
  macro in oniguruma which disables computed gotos, which are unsupported
  by clang-8 when compiling to wasm targets.
- Override `ONIG_EXTERN` to set function visibility for wasm builds. By
  default, functions are not exported under wasm targets. This flag
  ensures all functions in the static onig library are marked exported.
- Set `-fvisibility=default` for bindgen runs on wasm targets.