fix(build): Support FFMPEG_INCLUDE_DIR on Linux for hardsubx

The FFMPEG_INCLUDE_DIR environment variable was only checked inside
the macOS-specific block, so it had no effect on Linux builds.

Changes:
- Move FFMPEG_INCLUDE_DIR check outside platform-specific blocks so
  it works on all platforms
- Add pkg-config fallback on Linux to automatically find FFmpeg
  include paths

This fixes compilation on systems like Fedora where FFmpeg headers
are installed in non-standard locations (e.g., /usr/include/ffmpeg).

Fixes #1954

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: cfsmp3

src/rust/build.rs

@@ -84,7 +84,12 @@ fn main() {
     {
         builder = builder.clang_arg("-DENABLE_HARDSUBX");
 
-        // Add FFmpeg include paths for Mac
+        // Check FFMPEG_INCLUDE_DIR environment variable (works on all platforms)
+        if let Ok(ffmpeg_include) = env::var("FFMPEG_INCLUDE_DIR") {
+            builder = builder.clang_arg(format!("-I{}", ffmpeg_include));
+        }
+
+        // Add FFmpeg include paths for Mac (Homebrew)
         if cfg!(target_os = "macos") {
             // Try common Homebrew paths
             if std::path::Path::new("/opt/homebrew/include").exists() {
@@ -110,10 +115,14 @@ fn main() {
                     }
                 }
             }
+        }
 
-            // Also check environment variable
-            if let Ok(ffmpeg_include) = env::var("FFMPEG_INCLUDE_DIR") {
-                builder = builder.clang_arg(format!("-I{}", ffmpeg_include));
+        // On Linux, try pkg-config to find FFmpeg include paths
+        if cfg!(target_os = "linux") {
+            if let Ok(lib) = pkg_config::Config::new().probe("libavcodec") {
+                for path in lib.include_paths {
+                    builder = builder.clang_arg(format!("-I{}", path.display()));
+                }
             }
         }
     }