[libstdc++, patch] Fix build on APFS file system

[Mike Stump]

On Oct 18, 2017, at 7:51 AM, FX <fxcoudert@gmail.com> wrote:
> 
> Parallel builds of libstdc++ on APFS filesystem (with 1 ns granularity) on macOS 10.13 often fail (failure rate for “make -j2” to “make -j8” is about 60% from my own builds and results reported by others): https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81797
> This is reproducible with several versions of GNU make.
> 
> Changing libstdc++’s makefile to mark install-headers with .NOTPARALLEL fixes the issue. We've carried that patch in Homebrew (https://brew.sh) for a few months now, and have had no report of build issues since then.
> 
> Bootstrapped and regtested on x86_64-apple-darwin17 (as well as other platforms). OK to commit?

The patch seems like a rough bandaid to hide the real bug.  Better to identify the real bug.  If there is a missing dependency, then I'd like to think that adding the right dependency should resolve the issue.