add workflow to build mp4v2

Author: RaiaN

.github/workflows/README.md

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+# GitHub Workflows for Building MP4v2
+
+This directory contains GitHub Actions workflows for building the MP4v2 library for various platforms.
+
+## Available Workflows
+
+### 1. `build-mp4v2-linux-arm64.yml`
+**Purpose:** Build MP4v2 for Linux ARM64 using Autotools
+
+**Features:**
+- Uses QEMU for ARM64 emulation on x86_64 runners
+- Builds with GNU Autotools (./configure && make)
+- Automatically commits built libraries to the repository
+- Uploads artifacts for download
+
+**Triggers:**
+- Push to `main` or `master` branches
+- Pull requests
+- Manual workflow dispatch
+
+**Output:**
+- Static library: `Linux/arm64/libmp4v2.a`
+- Shared libraries: `Linux/arm64/libmp4v2.so*` (if built)
+- Headers: `include/mp4v2/`
+
+**Usage:**
+```bash
+# Triggered automatically on push, or run manually:
+# 1. Go to Actions tab
+# 2. Select "Build MP4v2 for Linux ARM64"
+# 3. Click "Run workflow"
+```
+
+---
+
+### 2. `build-mp4v2-cmake.yml`
+**Purpose:** Build MP4v2 for multiple platforms using CMake
+
+**Features:**
+- Supports Linux ARM64 and Linux x64
+- Uses CMake build system for more flexibility
+- Configurable platform selection via workflow inputs
+- Longer artifact retention (90 days)
+- Separate jobs for each platform
+
+**Triggers:**
+- Push to `main` or `master` branches (ARM64 only)
+- Pull requests (ARM64 only)
+- Manual workflow dispatch with platform selection
+
+**Output:**
+- Linux ARM64: `Linux/arm64/libmp4v2.a`
+- Linux x64: `Linux/x64/libmp4v2.a` (manual trigger only)
+- Headers: `include/mp4v2/`
+
+**Usage:**
+```bash
+# Manual trigger with options:
+# 1. Go to Actions tab
+# 2. Select "Build MP4v2 (CMake) for Multiple Platforms"
+# 3. Click "Run workflow"
+# 4. Enter platforms: "linux-arm64", "linux-x64", or "all"
+```
+
+---
+
+## Workflow Comparison
+
+| Feature | build-mp4v2-linux-arm64.yml | build-mp4v2-cmake.yml |
+|---------|-----------------------------|-----------------------|
+| Build System | Autotools | CMake |
+| Platforms | Linux ARM64 | Linux ARM64, x64 |
+| Auto Commit | Yes | No |
+| Artifact Retention | 30 days | 90 days |
+| Platform Selection | No | Yes (workflow_dispatch) |
+
+---
+
+## Common Tasks
+
+### Building for Linux ARM64
+Both workflows can build for Linux ARM64. The differences:
+- **Autotools workflow**: More traditional Unix build approach, auto-commits
+- **CMake workflow**: Modern build system, more control, no auto-commit
+
+### Downloading Artifacts
+After a workflow completes:
+1. Go to the workflow run page
+2. Scroll to the **Artifacts** section
+3. Download the ZIP file (e.g., `mp4v2-linux-arm64.zip`)
+4. Extract and copy files to your project
+
+### Modifying Build Options
+
+**For Autotools workflow**, edit the `./configure` line:
+```bash
+./configure --prefix=/build/install --enable-static --disable-shared
+```
+
+**For CMake workflow**, edit the `cmake` command:
+```bash
+cmake .. \
+  -DCMAKE_BUILD_TYPE=Release \
+  -DBUILD_SHARED_LIBS=OFF \
+  -DCMAKE_CXX_FLAGS="-O3"
+```
+
+---
+
+## Technical Details
+
+### QEMU Setup
+Both workflows use QEMU for ARM64 emulation:
+```yaml
+- name: Set up QEMU
+  uses: docker/setup-qemu-action@v3
+  with:
+    platforms: arm64
+```
+
+This allows building ARM64 binaries on x86_64 GitHub runners.
+
+### Docker Container
+ARM64 builds run in an `arm64v8/ubuntu:22.04` container:
+```bash
+docker run --rm --platform linux/arm64 \
+  -v ${{ github.workspace }}:/workspace \
+  arm64v8/ubuntu:22.04 bash -c "..."
+```
+
+### Verification
+After building, workflows verify the architecture:
+```bash
+file Linux/arm64/libmp4v2.a
+# Output: Linux/arm64/libmp4v2.a: current ar archive
+```
+
+---
+
+## Troubleshooting
+
+### Build Fails with "No such file or directory"
+- Check that the git clone succeeded
+- Verify the repository URL is correct
+- Ensure network connectivity during workflow run
+
+### Wrong Architecture Built
+- Verify `--platform linux/arm64` is set in docker run command
+- Check QEMU setup completed successfully
+- Run `file` command on the artifact to verify architecture
+
+### Artifact Not Found
+- Check the output path matches the artifact path in the workflow
+- Verify the build completed successfully
+- Check that `make install` or copy commands succeeded
+
+### Auto-commit Fails
+- Ensure the workflow has write permissions to the repository
+- Check that git config is set correctly
+- Verify `[skip ci]` is in the commit message to prevent infinite loops
+
+---
+
+## Adding New Platforms
+
+To add support for a new platform (e.g., Windows ARM64):
+
+1. Create a new job in the workflow:
+```yaml
+build-windows-arm64:
+  runs-on: windows-latest
+  steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+    # Add Windows-specific build steps
+```
+
+2. Use appropriate tools for that platform:
+   - Windows: Visual Studio, vcpkg
+   - macOS: Xcode command line tools
+   - Android: NDK
+
+3. Update artifact paths to match your repository structure
+
+---
+
+## Resources
+
+- **MP4v2 Repository:** https://github.com/enzo1982/mp4v2
+- **GitHub Actions Documentation:** https://docs.github.com/en/actions
+- **Docker QEMU Setup:** https://github.com/docker/setup-qemu-action
+- **Upload Artifacts:** https://github.com/actions/upload-artifact
+