Our build files were relying 'sysroot' to find gexiv2.h but this is
not possible with Apple Clang om which sysroot points to macOS SDK.
So, exotic environments like Homebrew were failing. Let's fix this.
This is probably a temporary fix until 13149 is tackled. And we need to make sure it works on all platforms.
The problem has been patched on macOS so we could also skip this change until the proper fix is done.
This explains such WARNING over a previous installation on Windows:
> (gimp-2.99.exe:11524): GLib-GObject-WARNING **: 21:36:36.664: Two different plugins tried to register 'ControllerDXDInput'.
The previous install was autotools-built, so it didn't have the typo.
