Bug 1617338 - Suppress warnings from third-party code in extensions/spellcheck/hunspell/glue/ r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D63749

--HG--
extra : moz-landing-system : lando

extensions/spellcheck/hunspell/glue/moz.build

@@ -34,3 +34,8 @@ EXPORTS.mozilla += [
      'RemoteSpellCheckEngineChild.h',
      'RemoteSpellCheckEngineParent.h',
 ]
+
+CXXFLAGS += [
+    # We force-include mozHunspellAllocator.h from third-party code.
+    '-Wno-undefined-var-template',
+]