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

[Jonathan Wakely]

On 18/10/17 16:51 +0200, FX 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?

Could you test using .PHONY: install-headers instead?

That target *is* phony, so telling make that seems sensible.

Presumably the same change is needed for install-freestanding-headers,
since it's very similar.