Fix old macOS Clang allocation flags - #278

Summary

Pass -fno-aligned-allocation when building against libc++ on macOS 10.12 and earlier with Clang.

Thanks to maintainers

Thanks for identifying the affected compiler and macOS boundary in #219, and for welcoming a focused fix.

Issue or motivation

Fixes #219. The oneTBB backend does not compile on the affected systems unless this flag is supplied.

Root cause

Darwin 16 and earlier use a libc++ implementation that needs aligned allocation disabled for the oneTBB build and for packages compiling against its headers. The flag was not carried through RcppParallel's configuration or downstream CxxFlags() output.

Change

• Detect the Darwin 16-or-earlier and clang++ combination during configuration.
• Pass the resulting flag to RcppParallel, the bundled oneTBB build, and downstream packages through CxxFlags().
• Add focused tests for the platform/compiler boundary and document the fix in NEWS.

Tests

• Platform helper regression checks for affected and unaffected combinations: passed
• R CMD build --no-manual: passed
• R CMD check --as-cran --no-manual: passed; all 13 test scripts passed, with only the existing development-version NOTE

Scope

This does not change flags on newer macOS releases, non-Clang compilers, or non-Darwin platforms. It does not modify the bundled oneTBB sources or alter unrelated compiler flags.