message: Display enabled features in the build log
diff: @@ -999,6 +999,23 @@ fn main() -> Result<()> {
  for &v in ENV_VARS.iter() {
  eprintln!("=== {} = {:?}", v, env::var_os(v));
  }
  + eprintln!("=== Enabled features:");
  + let features = env::vars()
  + .filter_map(|(mut name, val)| {
  + if val != "1" {
  + return None;
  + }
  + const PREFIX: &str = "CARGO_FEATURE_";
  + if name.starts_with(PREFIX) {
  + name.drain(..PREFIX.len());
  + Some(name)
  + } else {
  + None
  + }
  + });
  + for feature in features {
  + eprintln!("=== {}", feature);
  + }
  let opencv = if cfg!(feature = "docs-only") {
  Library::probe_from_paths("", "", "")?
  } else {
  