--- a/extensions/spellcheck/hunspell/glue/moz.build
+++ b/extensions/spellcheck/hunspell/glue/moz.build
@@ -6,22 +6,24 @@
 
 UNIFIED_SOURCES += [
     'mozHunspell.cpp',
     'mozHunspellDirProvider.cpp',
     'RemoteSpellCheckEngineChild.cpp',
     'RemoteSpellCheckEngineParent.cpp',
 ]
 
-CXXFLAGS += CONFIG['MOZ_HUNSPELL_CFLAGS']
-
 FINAL_LIBRARY = 'xul'
 
+if CONFIG['MOZ_SYSTEM_HUNSPELL']:
+    CXXFLAGS += CONFIG['MOZ_HUNSPELL_CFLAGS']
+else:
+    LOCAL_INCLUDES += ['../src']
+
 LOCAL_INCLUDES += [
-    '../src',
     '/dom/base',
     '/extensions/spellcheck/src',
 ]
 
 include('/ipc/chromium/chromium-config.mozbuild')
 
 IPDL_SOURCES = [
     'PRemoteSpellcheckEngine.ipdl',

--- a/extensions/spellcheck/src/moz.build
+++ b/extensions/spellcheck/src/moz.build
@@ -12,19 +12,23 @@ UNIFIED_SOURCES += [
     'mozPersonalDictionary.cpp',
     'mozSpellChecker.cpp',
     'mozSpellCheckerFactory.cpp',
     'mozSpellI18NManager.cpp',
 ]
 
 FINAL_LIBRARY = 'xul'
 
+if CONFIG['MOZ_SYSTEM_HUNSPELL']:
+    CXXFLAGS += CONFIG['MOZ_HUNSPELL_CFLAGS']
+else:
+    LOCAL_INCLUDES += ['../hunspell/src']
+
 LOCAL_INCLUDES += [
     '../hunspell/glue',
-    '../hunspell/src',
     '/dom/base',
 ]
 EXPORTS.mozilla += [
      'mozSpellChecker.h',
 ]
 
 if CONFIG['GNU_CXX']:
     CXXFLAGS += ['-Wno-error=shadow']