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>

scons/gallium.py

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