Fix C++ runtime detection.

- Don't try to detect libc++abi.  It doesn't actually work, so don't
   try it.
 - Correctly try to link the standard library.

This should fix the build on GNU/Linux systems with libc++abi.so
installed (including our Ubuntu CI!).

CMake/CMakeLists.txt

@@ -2,7 +2,7 @@ cmake_minimum_required(VERSION 3.1)
 project(test_cxx_runtime)
 
 add_executable(test_cxx_runtime typeinfo_test.cc)
-set(CMAKE_CXX_IMPLICIT_LINK_LIBRARIES "")
+add_executable(test_cxx_stdlib typeinfo_test.cc)
 if (CXX_RUNTIME)
 	target_link_libraries(test_cxx_runtime ${CXX_RUNTIME})
 	set_target_properties(test_cxx_runtime PROPERTIES

CMakeLists.txt

@@ -284,8 +284,9 @@ if (ENABLE_OBJCXX)
 		if (NOT CXX_RUNTIME)
 			test_cxx(supc++ false)
 		endif (NOT CXX_RUNTIME)
+		# libc++abi does not currently work, don't try it.
 		if (NOT CXX_RUNTIME)
-			test_cxx(c++abi false)
+			# test_cxx(c++abi false)
 		endif (NOT CXX_RUNTIME)
 
 		# If we have a C++ ABI library, then we can produce a single libobjc that
@@ -295,10 +296,11 @@ if (ENABLE_OBJCXX)
 			message(STATUS "Using ${CXX_RUNTIME} as the C++ runtime library")
 		else()
 			message(STATUS "Testing C++ standard library")
+			message(STATUS "Using ${CMAKE_C_COMPILER}")
 			try_compile(USERUNTIME 
 				"${CMAKE_BINARY_DIR}/CMake"
 				"${CMAKE_CURRENT_LIST_DIR}/CMake"
-				test_cxx_runtime)
+				test_cxx_stdlib)
 			if (${USERUNTIME})
 				message(STATUS "libobjc will depend on C++ standard library")
 				set(CXXRT_IS_STDLIB true)