Tests: Fix logic to enable RunCMake.try_compile OBJC/OBJCXX cases

CMake itself does not enable the OBJC/OBJCXX languages, so the
`CMAKE_OBJ{C,CXX}_STANDARD_DEFAULT` variables are never defined.
Detect them inside the test instead.

Tests/RunCMake/CMakeLists.txt

@@ -557,9 +557,8 @@ function(add_RunCMake_test_try_compile)
       CMake_TEST_CUDA
       CMake_TEST_ISPC
       CMake_TEST_HIP
+      CMake_TEST_OBJC
       CMake_TEST_FILESYSTEM_1S
-      CMAKE_OBJC_STANDARD_DEFAULT
-      CMAKE_OBJCXX_STANDARD_DEFAULT
     )
     if(DEFINED ${var})
       list(APPEND try_compile_ARGS -D${var}=${${var}})

Tests/RunCMake/try_compile/Inspect.cmake

@@ -1,4 +1,18 @@
 enable_language(CXX)
-file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/info.cmake" "
-set(CMAKE_CXX_EXTENSIONS_DEFAULT \"${CMAKE_CXX_EXTENSIONS_DEFAULT}\")
-")
+if(CMake_TEST_OBJC)
+  enable_language(OBJC)
+  enable_language(OBJCXX)
+endif()
+
+set(info "")
+foreach(var
+    CMAKE_CXX_EXTENSIONS_DEFAULT
+    CMAKE_OBJC_STANDARD_DEFAULT
+    CMAKE_OBJCXX_STANDARD_DEFAULT
+    )
+  if(DEFINED ${var})
+    string(APPEND info "set(${var} \"${${var}}\")\n")
+  endif()
+endforeach()
+
+file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/info.cmake" "${info}")

Tests/RunCMake/try_compile/RunCMakeTest.cmake

@@ -1,5 +1,14 @@
 include(RunCMake)
 
+# Detect information from the toolchain:
+# - CMAKE_CXX_EXTENSIONS_DEFAULT
+# - CMAKE_OBJC_STANDARD_DEFAULT
+# - CMAKE_OBJCXX_STANDARD_DEFAULT
+run_cmake_with_options(Inspect
+  -DCMake_TEST_OBJC=${CMake_TEST_OBJC}
+  )
+include("${RunCMake_BINARY_DIR}/Inspect-build/info.cmake")
+
 run_cmake(NoArgs)
 run_cmake(OneArg)
 run_cmake(TwoArgs)
@@ -88,12 +97,6 @@ if(RunCMake_GENERATOR MATCHES "Make|Ninja")
   unset(RunCMake_TEST_NO_CLEAN)
 endif()
 
-# Lookup CMAKE_CXX_EXTENSIONS_DEFAULT.
-# FIXME: Someday we could move this to the top of the file and use it in
-# place of some of the values passed by 'Tests/RunCMake/CMakeLists.txt'.
-run_cmake(Inspect)
-include("${RunCMake_BINARY_DIR}/Inspect-build/info.cmake")
-
 # FIXME: Support more compilers and default standard levels.
 if (DEFINED CMAKE_CXX_STANDARD_DEFAULT AND
     DEFINED CMAKE_CXX_EXTENSIONS_DEFAULT AND (