Constify AV_CODEC_ID_FIRST_* in enums

There are dummy IDs AV_CODEC_ID_FIRST_AUDIO, AV_CODEC_ID_FIRST_SUBTITLE, and
AV_CODEC_ID_FIRST_UNKNOWN that are duplicates of their following entries in the
AVCodecID enum in libavcodec/avcodec.h. In C this is not a problem, but when
bindgen converts the C enum to Rust enum it has to drop one in each pair of
duplicates, and since the dummy IDs are listed first, they push the meaningful
ones AV_CODEC_ID_FIRST_AUDIO, AV_CODEC_ID_DVD_SUBTITLE, and AV_CODEC_ID_TTF to
associated constants. We introduce enum_variant_behavior callback to fix this.

sys/build.rs

@@ -12,7 +12,7 @@ use std::process::Command;
 use std::str;
 
 use regex::Regex;
-use bindgen::callbacks::{IntKind, ParseCallbacks, MacroParsingBehavior};
+use bindgen::callbacks::{EnumVariantCustomBehavior, EnumVariantValue, IntKind, ParseCallbacks, MacroParsingBehavior};
 
 #[derive(Debug)]
 struct Library {
@@ -43,9 +43,9 @@ static LIBRARIES: &[Library] = &[
 ];
 
 #[derive(Debug)]
-struct IntCallbacks;
+struct Callbacks;
 
-impl ParseCallbacks for IntCallbacks {
+impl ParseCallbacks for Callbacks {
     fn int_macro(&self, _name: &str, value: i64) -> Option<IntKind> {
         let ch_layout = Regex::new(r"^AV_CH").unwrap();
         let codec_cap = Regex::new(r"^AV_CODEC_CAP").unwrap();
@@ -72,6 +72,20 @@ impl ParseCallbacks for IntCallbacks {
         }
     }
 
+    fn enum_variant_behavior(
+        &self,
+        _enum_name: Option<&str>,
+        original_variant_name: &str,
+        _variant_value: EnumVariantValue,
+    ) -> Option<EnumVariantCustomBehavior> {
+        let dummy_codec_id = Regex::new(r"^AV_CODEC_ID_FIRST").unwrap();
+        if dummy_codec_id.is_match(original_variant_name) {
+            Some(EnumVariantCustomBehavior::Constify)
+        } else {
+            None
+        }
+    }
+
     // https://github.com/rust-lang/rust-bindgen/issues/687#issuecomment-388277405
     fn will_parse_macro(&self, name: &str) -> MacroParsingBehavior {
         use MacroParsingBehavior::*;
@@ -1035,7 +1049,7 @@ fn main() {
         .prepend_enum_name(false)
         .derive_eq(true)
         .size_t_is_usize(true)
-        .parse_callbacks(Box::new(IntCallbacks));
+        .parse_callbacks(Box::new(Callbacks));
 
     // The input headers we would like to generate
     // bindings for.