Use Homebrew include directory for pcre2.h on macOS if necessary

[madmajestro]

When pcre2 is installed via Homebrew on macOS, the header file pcre2.h is typically located in the Homebrew include directory. This leads to build errors since the compiler does not use this directory by default. Therefore, the Homebrew include directory is now added as include directory for the compiler if it contains pcre2.h. In addition, the CI has been extended to check the build on macOS.

Closes #572

[nikic]

Can this cause issues with wrong headers being used if pcre2 was not installed via Homebrew (but a Homebrew installation happens to exist)?

[madmajestro]

I’ve thought about that, too. I consider it quite unlikely, but possible. I’ll take a closer look at it. It probably makes sense to check harder whether adding this include path is really necessary.

[madmajestro]

I have now revised this a bit. The directory is now added only if the compiler cannot find the file in the standard include directories and if it exists in the Homebrew include directory instead.

[madmajestro]

I think this approach should work well. I plan to run a few more tests and merge it in a couple of days. If you have any concerns, please let me know.

[nikic]

LGTM

[madmajestro]

While testing, I found another issue:
If APCu is built in-tree and the PHP build system uses the built-in PCRE library, the new logic is now skipped because the external pcre2.h is not used in this case.