GNU: C++23 support

Added in GCC commit 78739c2df788ee5c868d998a6333d453317d8711, released in 11.1.
No lettered variant beforehand this time around.

Implements #22139.
36 jobs for !6108 with cpp23_gcc in 6 minutes and 53 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Build
manual #5776290
linux docker build cmake manual
build:centos6-x86_64
passed #5776287
linux docker build cmake
build:debian10-iwyu

00:02:43

manual #5776291
linux-3.17 linux docker build cmake manual
build:fedora33-ninja
passed #5776289
linux-3.17 linux docker build cmake
build:fedora33-sphinx

00:05:58

passed #5776288
linux-3.17 linux docker build cmake
build:fedora33-tidy

00:06:53

manual #5776293
nonconcurrent xcode-12.4 shell macos cmake manual
build:macos-x86_64-makefiles
manual #5776292
cmake nonconcurrent xcode-12.4 shell macos manual
build:macos-x86_64-ninja
manual #5776294
nonconcurrent msvc-19.28-16.9 vs2019 shell windows cmake manual
build:windows-vs2019-x64-ninja
 
  Test
created #5776295
x11 linux-3.17 linux docker cmake
test:fedora33-ninja
created #5776297
nonconcurrent xcode-12.4 shell macos cmake
test:macos-x86_64-makefiles
created #5776296
nonconcurrent xcode-12.4 shell macos cmake
test:macos-x86_64-ninja
created #5776298
nonconcurrent msvc-19.28-16.9 vs2019 shell windows cmake
test:windows-vs2019-x64-ninja
 
  Test Ext
created #5776301
linux docker cuda-rt cmake
test:cuda10.2-nvidia
created #5776299
linux-3.17 linux docker build cmake
test:debian10-ninja
created #5776300
linux-3.17 linux docker build cmake
test:fedora33-makefiles
created #5776302
linux-3.17 linux docker build cmake
test:fedora33-ninja-multi
created #5776303
concurrent xcode-12.4 shell macos cmake
test:macos-x86_64-xcode
created #5776304
concurrent msvc-19.28-16.9 vs2019 shell windows cmake
test:windows-vs2019-x64
 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed cdash

passed cdash

passed cdash

passed cdash

passed ghostflow-check-master

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager