Re: [PATCH] audo-detect and use -moutline-atomics compilation flag for aarch64

> I will investigate the autoconf issue and send you a patch specific to PG10 and 9.
> Most likely the PGAC_PROG_CXX_CFLAGS_OPT macro has been introduced after these releases.

The above macro has been added in March 2018:
https://github.com/postgres/postgres/commit/6869b4f2584787d9e4cefaab8a4bae1ecbe63766
the patch also adds CXXFLAGS, and so before this patch there is no need to detect CXXFLAGS.

Please see attached a patch tested on Ubuntu 20.04 on the following branches:
remotes/origin/REL9_6_STABLE
remotes/origin/REL_10_STABLE

I checked that REL_11_STABLE has the CXXFLAGS and the above macro.

Thanks,
Sebastian