Make sure --enable-system-hunspell picks up system headers

--- extensions/spellcheck/hunspell/glue/moz.build
+++ extensions/spellcheck/hunspell/glue/moz.build
@@ -11,12 +11,14 @@ UNIFIED_SOURCES += [
     'RemoteSpellCheckEngineParent.cpp',
 ]
 
-CXXFLAGS += CONFIG['MOZ_HUNSPELL_CFLAGS']
-
 FINAL_LIBRARY = 'xul'
 
+if CONFIG['MOZ_NATIVE_HUNSPELL']:
+    CXXFLAGS += CONFIG['MOZ_HUNSPELL_CFLAGS']
+else:
+    LOCAL_INCLUDES += ['../src']
+
 LOCAL_INCLUDES += [
-    '../src',
     '/dom/base',
     '/extensions/spellcheck/src',
 ]
--- extensions/spellcheck/src/moz.build
+++ extensions/spellcheck/src/moz.build
@@ -17,9 +17,13 @@ UNIFIED_SOURCES += [
 
 FINAL_LIBRARY = 'xul'
 
+if CONFIG['MOZ_NATIVE_HUNSPELL']:
+    CXXFLAGS += CONFIG['MOZ_HUNSPELL_CFLAGS']
+else:
+    LOCAL_INCLUDES += ['../hunspell/src']
+
 LOCAL_INCLUDES += [
     '../hunspell/glue',
-    '../hunspell/src',
     '/dom/base',
 ]
 EXPORTS.mozilla += [
