Tests: Update RunCMake.CompileFeatures for Xcode 15.3

Xcode no longer puts an extra backslash in its `-std=` flags.

Tests/RunCMake/CompileFeatures/RunCMakeTest.cmake

@@ -62,7 +62,7 @@ macro(mangle_flags variable)
 
   if(RunCMake_GENERATOR MATCHES "Visual Studio" AND MSVC_TOOLSET_VERSION GREATER_EQUAL 141)
     string(REPLACE "-" "/" result "${result}")
-  elseif(RunCMake_GENERATOR STREQUAL "Xcode" AND CMAKE_XCODE_BUILD_SYSTEM GREATER_EQUAL 12)
+  elseif(RunCMake_GENERATOR STREQUAL "Xcode" AND XCODE_VERSION VERSION_LESS 15.3 AND CMAKE_XCODE_BUILD_SYSTEM GREATER_EQUAL 12)
     string(REPLACE "=" [[\\=]] result "${result}")
   endif()
 

Tests/RunCMake/CompileFeatures/compiler_introspection.cmake

@@ -16,6 +16,13 @@ set(CMAKE_XCODE_BUILD_SYSTEM ${CMAKE_XCODE_BUILD_SYSTEM})
 ")
 endif()
 
+if(XCODE_VERSION)
+  string(APPEND info "
+set(XCODE_VERSION ${XCODE_VERSION})
+
+")
+endif()
+
 macro(info lang)
   string(APPEND info "\
 set(${lang}_STANDARD_DEFAULT ${CMAKE_${lang}_STANDARD_DEFAULT})