meson: Don't depend on expat for !use_xmlconfig

If we don't use xmlconfig, expat becomes an optional dependency on
Linux/BSD, which may be convenient for embedded systems that do not
otherwise need expat. (expat is so ubiquitous that this
probably doesn't matter a ton in practice, but fewer required Mesa deps
is probably nice anyway.)

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Suggested-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19626>

meson.build

@@ -1633,13 +1633,21 @@ if dep_thread.found()
     pre_args += '-DHAVE_PTHREAD_SETAFFINITY'
   endif
 endif
+
+# We don't have expat on Android or Windows, which is a needed dep for xmlconfig
+supports_xmlconfig = not (with_platform_android or with_platform_windows)
+
+opt_xmlconfig = get_option('xmlconfig')
+if opt_xmlconfig.enabled() and not supports_xmlconfig
+   error('xmlconfig not available on Android or Windows')
+endif
+use_xmlconfig = supports_xmlconfig and not opt_xmlconfig.disabled()
+
 if host_machine.system() == 'darwin'
   dep_expat = meson.get_compiler('c').find_library('expat')
-elif host_machine.system() != 'windows'
-  dep_expat = dependency('expat', fallback : ['expat', 'expat_dep'],
-                         required: not with_platform_android)
 else
-  dep_expat = null_dep
+  dep_expat = dependency('expat', fallback : ['expat', 'expat_dep'],
+                         required: use_xmlconfig)
 endif
 # Predefined macros for windows
 if host_machine.system() == 'windows'