meson: don't check for expat if we've disabled it

The current behavior results in a fallback to building our own expat if
it's not found, even in cases where it's been explicitly been disabled.
This has been fixed on main, but that patch is part of a refactor that
updates the required meson version, and isn't suitable for backport.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/8259
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21220>
1 file changed