cmake: allow C-only builds when tests are disabled

[fhgffy]

Description

The library and ordinary examples are C, but project(... LANGUAGES C CXX) requires a working C++ compiler even with WITH_TESTS=OFF. For example, configuration fails on master with:

cmake -S . -B build-c-only -DWITH_TESTS=OFF \
  -DCMAKE_CXX_COMPILER=/nonexistent/cxx

Enable CXX only for the optional test suite or CMake older than 3.12. The older-CMake exception preserves the existing minimum version: GenerateExportHeader gained C-project support in CMake 3.12.

Add a standalone regression project that forces an unavailable C++ compiler, builds libcbor and its examples, checks that CXX was not enabled, and runs the existing C linkage consumer. Run it from the existing CircleCI linkage-test command. Document the C++ requirement and add a changelog entry.

Validation

Tested on Linux with GCC/G++ 14.2.0:

• The new regression fails against the unmodified base for the unavailable C++ compiler and passes with the fix on CMake 3.31.6 and 4.0.3
• Static and shared C-only builds, examples, and installed CMake consumers pass
• All 29 upstream tests pass on the base, fixed static build, and fixed shared build; both C++ linkage controls pass
• Ordinary builds with CMake 3.11.2 still pass with C++ available, using explicit C99 to avoid an unrelated old-CMake/GCC-14 standard-detection mismatch
• The exact added CircleCI command passes locally, including paths containing spaces
• Independent review and a fresh offline replay of the final patch pass

CMake 3.5, exact 3.12, Windows/macOS, sanitizers, documentation generation, lint tools, and the full CI matrix were not run locally.

Checklist

• Contribution guidance reviewed
• Regression test added
• Documentation updated
• CHANGELOG updated without changing the version

No public API or runtime C implementation changes, new platform-specific code, or intended security/performance changes.

Developed with AI assistance; the patch was independently reviewed and tested locally.