Feature-detect GLOB_BRACE for musl-libc PHP builds (Alpine, static FrankenPHP)

[superdav42]

Summary

Wpup_UpdateServer::findFirstAsset() references GLOB_BRACE unconditionally:

https://github.com/YahnisElsts/wp-update-server/blob/master/includes/Wpup/UpdateServer.php#L394

GLOB_BRACE is not defined on PHP builds linked against musl libc — for example Alpine container images and the official static FrankenPHP binary releases (see PHP manual note: "GLOB_BRACE is not available on some non-GNU systems, like Solaris or Alpine Linux").

In PHP 8 referencing an undefined constant is a fatal Error (not the PHP 7 warning), so every call to findFirstAsset() crashes the update API response when it tries to resolve a plugin banner or icon:

PHP Fatal error:  Uncaught Error: Undefined constant "GLOB_BRACE"
  in .../includes/Wpup/UpdateServer.php:394

Change

Switch the array-of-extensions branch to runtime feature detection:

• When GLOB_BRACE is defined, behaviour is identical to before (single glob() with the brace pattern).
• When it is missing, fall back to one glob() call per extension and merge results.
• The string-extension branch no longer passes GLOB_BRACE since it was meaningless without braces.

Reproduce

# On any musl-libc PHP build (Alpine, static FrankenPHP):
php -r 'var_dump(defined("GLOB_BRACE"));'   # => bool(false)
# Hit any update API endpoint that resolves a banner/icon — fatal in error log.

Verification

• php -l clean.
• Smoke test exercising all branches (array+brace, array+fallback, string-extension, no-match) returns identical results under glibc system PHP and musl FrankenPHP.
• Tested on a live install (FrankenPHP v1.4.4, PHP 8.4.5, static musl build): update API requests now respond normally; previously each one fatal-ed.

Notes

No behavioural change on glibc systems. No new dependencies. No public API changes.

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