build: make CXX overridable (CXX ?= g++) for clang builds

[renecannao]

Summary

Makefile hardcoded CXX = g++. A CXX=clang++ make environment prefix does not override a Makefile = assignment, so downstream consumers that build ParserSQL with clang in containers shipping only clang/clang++ (no g++) failed with exit 127 (g++: command not found).

Changing to CXX ?= g++ lets the environment-provided compiler win while keeping g++ as the default.

Impact

Surfaced building ProxySQL 3.0.9/3.1.9 clang packages (16 distro variants failed). Cut as v1.0.9 and re-vendored into ProxySQL.

Test

CXX=clang++ make lib now compiles with clang++ (previously ignored).

Summary by CodeRabbit

• Chores
  • Improved build system configuration to respect externally-provided C++ compiler settings. The build process now supports custom compiler selection via environment variables and command-line parameters, enabling greater flexibility for developers and automated build systems. This enhancement improves compatibility across different development environments.

[coderabbitai]

Caution

Review failed

Pull request was closed or merged during review

📝 Walkthrough

Walkthrough

The Makefile's C++ compiler variable assignment is updated from unconditional to conditional (CXX ?=), allowing external CXX values from environment variables or command-line arguments to override the default g++ instead of being forcibly replaced.

Changes

Build Configuration Flexibility

Layer / File(s)	Summary
Conditional compiler variable default Makefile	The CXX variable assignment is changed from CXX = g++ to CXX ?= g++, enabling external CXX environment or command-line overrides instead of forcing the default compiler.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

🐰 A Makefile change, oh so slight,
One operator shift, a build-time delight!
From = to ?=, now external tools reign,
Compilers choose freely, no override pain!
Flex and compose, let builders decide. 🛠️

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly and accurately summarizes the main change: making the CXX variable overridable in the Makefile using conditional assignment, enabling clang builds.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/makefile-cxx-conditional

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