Support enum forward decls in Objective C++
NS_ENUM changes defintion in Objective C++ based on the C++ spec being
compiled with, special case the one situation where it wouldn't support doing a
forward decl for the enum.
diff --git a/Makefile.am b/Makefile.am
index 8b36178..6d91808 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -456,6 +456,7 @@
objectivec/Tests/GPBMessageTests+Runtime.m \
objectivec/Tests/GPBMessageTests+Serialization.m \
objectivec/Tests/GPBMessageTests.m \
+ objectivec/Tests/GPBObjectiveCPlusPlusTest.mm \
objectivec/Tests/GPBPerfTests.m \
objectivec/Tests/GPBStringTests.m \
objectivec/Tests/GPBSwiftTests.swift \