libgccjit installed via Homebrew now uses the correct .dylib dynamic library
extension on macOS. This causes configure to fail because it is still
using the .so extension.

The attached patch fixes the libgccjit library extension in configure.ac

In GNU Emacs 28.1.50 (build 2, x86_64-apple-darwin21.4.0, NS appkit-2113.40
Version 12.3.1 (Build 21E258))
 of 2022-04-25 built on iMac-Pro.local
Repository revision: 7c1fc2348918a1ad8c61c7519a51042d48d7aa8d
Repository branch: emacs-28
Windowing system distributor 'Apple', version 10.3.2113
System Description:  macOS 12.3.1

Configured using:
 'configure --with-native-compilation'

Configured features:
ACL GMP GNUTLS JSON LIBXML2 MODULES NATIVE_COMP NOTIFY KQUEUE NS PDUMPER
THREADS TOOLKIT_SCROLL_BARS ZLIB

Important settings:
  value of $LANG: en_GB.UTF-8
  locale-coding-system: utf-8-unix