run_project_tests: correctly search for objcpp compiler

Instead of looking for an objc compiler. Fixes objc++ tests on
DragonFlyBSD (which has an objc but not an objc++ compiler)

Really though, the objc and objc++ tests need to be untangled so that
the objc tests can run even if an objc++ compiler is unavilable.

run_project_tests.py

@@ -453,7 +453,7 @@ def have_objc_compiler():
             return False
         try:
             objc_comp.sanity_check(env.get_scratch_dir(), env)
-            objcpp_comp = env.detect_objc_compiler(False)
+            objcpp_comp = env.detect_objcpp_compiler(False)
         except:
             return False
         if not objcpp_comp: