Improved OpenMP setup and detection (closes #493)

[eddelbuettel]

No description provided.

[Copilot]

Pull Request Overview

This PR simplifies the OpenMP detection and configuration logic by removing compiler-specific version checks and relying more on direct OpenMP compilation tests. The changes streamline the configure script to test OpenMP support directly rather than inferring support from compiler versions.

Key Changes:

• Removed complex compiler version detection logic (g++, clang) and replaced with direct OpenMP compilation test
• Consolidated OpenMP flag and define settings based on compilation test results
• Updated CI workflow to test on both container and macOS environments

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
configure.ac	Removed ~100 lines of compiler-specific version checks; now sets OpenMP variables directly from compilation test
configure	Generated configure script reflecting the simplified configure.ac logic
src/Makevars.in	Removed blank line
src/Makevars.win	Removed blank line
inst/NEWS.Rd	Added changelog entry about simplified OpenMP detection
ChangeLog	Added dated entry documenting the OpenMP setup changes
.github/workflows/ci.yaml	Added container-based testing and macOS testing with configure output

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