Add -Wno-implicit-fallthrough flags conditionally

[petk]

Older GCC versions (< 7.0) don't support the -Wno-implicit-fallthrough compiler flag. This adds the flag conditionally in case some other compiler will run into same issue.

Fixes #13330

[Girgias]

RHEL 7 is EOL in June of this year.

Does it really make sense to fix this?

Also the main reason for this flag, other than opcache, is because we don't control the bundle libraries. For opcache I don't remember the reason, so it might make sense to remove the flag and see what happens again.

[devnexen]

I m a tad on the fence for the change too.

• mac 10.6 last update was around 2011-ish and gcc 4.2 is very old.
• The bugtracker mentions php 8.1 they will have to patch this version anyway.
• but in another hand, the change is not too big so I do not really object. I just hope we won t get more demands in that direction..

[Girgias]

I don't mind this change either, but considering that GCC 8.1 supports C17 and RHEL 8 comes with GCC 8.x I would imagine that we might bump our minimum C requirement soon anyway (mainly for the atomics supports I'd gather).

[petk]

Hm, these flags are difficult :D I'll wait a bit with this. I've checked some of the compilers and basically all decent compilers today should support this flag. Even Solaris Studio compiler, Intel compiler, and MS Visual Studio with 2019 in its /wd and number form support it. That's good to know. I understand now...

[ryandesign]

I wish you would merge this PR so that I wouldn't have to keep maintaining our patch.

[petk]

Sure, let's add this conditionally then. Added to PHP-8.2 and up.

In the upcoming PHP-8.4 this compile flag was even reduced down to the minimum where it is still needed. The ext/date is still pending the merge, so somewhere right after the PHP 8.4 #14187