Fix segfault and enhance build system with comprehensive CI

[cofyc]

Summary

This PR implements key improvements to enhance the reliability, usability, and testing of the argparse library:

🔧 Critical Bug Fix

• Fix segmentation fault risk in argparse_usage() when options->help is NULL
• Add comprehensive null pointer safety checks

🚀 Enhanced Build System

• Add installation support to both Makefile and CMake with configurable PREFIX
• Create pkg-config file template for easy library integration
• Support for make install, make uninstall, and cmake --install

🧪 Comprehensive Test Suite

Added 5 new test cases covering:

• NULL help text handling (prevents segfaults)
• Callback functionality with multiple scenarios
• Option groups display formatting
• ARGPARSE_IGNORE_UNKNOWN_ARGS flag behavior
• Stress testing with 71 options to verify scalability

🏗️ CI/CD Infrastructure

• Dual build system CI: Tests both Make and CMake builds
• Multi-platform support: Ubuntu, macOS, Windows
• Memory safety testing: AddressSanitizer, UBSan, Valgrind
• Cross-platform compatibility: Tests C89, C99, C11 with gcc/clang
• Installation verification: Tests both installation methods

📚 Documentation & Development

• Add CLAUDE.md with project guidance and build instructions
• Configure development workflow settings

Test Results

All 57 tests pass including the new comprehensive test suite:

Files=8, Tests=57, 1 wallclock secs
Result: PASS

Addresses Issues

• Potential segfault when displaying help with NULL descriptions
• Lack of installation support for downstream users
• Missing pkg-config integration
• Limited test coverage for edge cases
• No CI infrastructure for both build systems

🤖 Generated with Claude Code