ffmpeg: fix build failure with gbk encoding

[choyy]

fix #6167, and maybe #8499

[gemini-code-assist]

Summary of Changes

Hello @choyy, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a persistent build failure in FFmpeg when compiled within a MinGW64 environment, specifically targeting issues arising from config.h files encoded in GBK or similar Chinese character sets. The solution involves integrating the uchardet tool into the MSYS2 build environment and then using it to automatically detect and convert problematic config.h files to UTF-8 with a Byte Order Mark (BOM) before the FFmpeg autoconf installation step. This ensures consistent encoding and prevents build interruptions, ultimately fixing issues #6167 and potentially #8499.

Highlights

• Encoding Fix for FFmpeg Build: Implemented a mechanism to detect and convert GBK-encoded config.h files to UTF-8 with a Byte Order Mark (BOM) during the FFmpeg build process on MinGW64, resolving build failures.
• uchardet Integration: Integrated the uchardet tool into the MSYS2 package configuration, making it available for character encoding detection in the build environment.
• Dependency Management: Added uchardet as a required configuration for FFmpeg builds under MinGW64 to facilitate the encoding conversion.

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[gemini-code-assist]

Code Review

This pull request addresses a build failure for ffmpeg on systems with GBK encoding by introducing uchardet to detect and convert a configuration file to UTF-8. The changes are logical and well-targeted. I've provided a couple of suggestions for improvement: one to ensure the fix is applied consistently across all Windows build paths, and another to improve the robustness of the shell script by quoting a file path variable. Overall, this is a good fix for the reported issue.

[gemini-code-assist]

This is a great fix for the encoding issue. However, this io.replace call is inside an if path.cygwin then block. There is another code path for Windows builds in the else block (starting on line 283) which also invokes the ./configure script. This fix should also be applied there to ensure consistent behavior for all Windows builds. You could move this block of code to before the if path.cygwin then check (e.g., after line 274) to apply it to both paths.

[gemini-code-assist]

For robustness, it's a good practice to quote the $TMPH variable in shell scripts to handle potential spaces or special characters in the file path.

                    config_encodings=$(uchardet "$TMPH")
                    case "$config_encodings" in GB18030|GBK|GB2312)
                        { printf '\xEF\xBB\xBF'; iconv -f "$config_encodings" -t UTF-8 -c "$TMPH"; } > config.h;; 
                    *) cp_if_changed "$TMPH" config.h;;