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                                       \