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>
(cherry picked from commit bf8b5de6b9)

meson.build

@@ -250,6 +250,7 @@ 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

scons/gallium.py

@@ -352,6 +352,7 @@ def generate(env):
                 '_DARWIN_C_SOURCE',
                 'GLX_USE_APPLEGL',
                 'GLX_DIRECT_RENDERING',
+                'BUILDING_MESA',
             ]
         else:
             cppdefines += [