scons+meson: suppress spammy build warning on MacOS

Originally introduced in c7f3657450 ("darwin: Suppress type
conversion warnings for GLhandleARB") to fix Bugzilla #66346 [1], this
workaround was never ported to Scons or Meson.

[1] https://bugs.freedesktop.org/66346

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

meson.build

@@ -252,6 +252,7 @@ endif
 
 if host_machine.system() == 'darwin'
   with_dri_platform = 'apple'
+  pre_args += '-DBUILDING_MESA'
 elif ['windows', 'cygwin'].contains(host_machine.system())
   with_dri_platform = 'windows'
 elif system_has_kms_drm