Fix Permission denied error when building installed packages

[terasakisatoshi]

Summary

This PR fixes a critical issue where Pkg.build() fails with a Permission denied error when the package is installed via Pkg.add() from a Git repository. The error occurs because the build script tries to write C_API.jl directly to the src/ directory, which is read-only in installed packages.

Problem

When installing the package from GitHub using:

Pkg.add(url="...", subdir="CalcPi.jl")
Pkg.build("CalcPi")

The build process fails with:

ERROR: LoadError: SystemError: opening file "./../src/C_API.jl": Permission denied

This happens because generate_C_API.jl tries to write directly to src/C_API.jl, which is read-only in the installed package directory.

Solution

1. Added --output-dir option to generate_C_API.jl

   • Allows specifying a custom output directory for the generated file
   • Improves argument parsing to handle multiple options correctly

2. Modified build.jl to use temporary directory

   • Generates C_API.jl in a temporary directory first
   • Attempts to copy to src/ if writable (development environment)
   • Gracefully handles read-only src/ for installed packages
   • Shows appropriate warnings when src/ is not writable

3. Added test script

   • run.sh script to test package installation from GitHub
   • Useful for verifying that Pkg.add() and Pkg.test() work correctly

Changes

• CalcPi.jl/utils/generate_C_API.jl: Added --output-dir option
• CalcPi.jl/deps/build.jl: Use temporary directory and handle read-only src/
• run.sh: Test script for package installation

Benefits

• Fixes build failures for installed packages
• Maintains compatibility with development workflow
• Provides better error handling and user feedback
• Allows testing package installation process

Testing

• Tested in development environment (writable src/)
• Tested with installed package (read-only src/)
• Verified that existing C_API.jl is used when src/ is read-only