Fix aligned allocation for old macOS targets

[kevinushey]

Summary

• Detect Clang, libc++, and a deployment target earlier than macOS 10.13 with an actual R toolchain probe.
• Pass -fno-aligned-allocation to RcppParallel, the bundled oneTBB build, and downstream packages through CxxFlags().
• Add focused downstream-flag coverage and document the fix in NEWS.

The probe uses R CMD SHLIB so it follows the compiler, standard library, launchers, deployment flags, and user Makevars settings that will actually build the package.

Fixes #219.
Alternative implementation of #278.

Testing

• Normal configure omits the workaround.
• MACOSX_DEPLOYMENT_TARGET=10.12 configure emits the workaround in all required locations.
• R CMD build --no-build-vignettes --no-manual
• Source installation from the generated tarball
• Focused test-cxx-flags.R regression test
• R CMD check --no-tests --no-manual --no-build-vignettes: OK