Ninja: Record showIncludes detection in configure log

Also avoid running the detection multiple times.

Modules/CMakeDetermineCompilerId.cmake

@@ -219,6 +219,9 @@ function(CMAKE_DETERMINE_COMPILER_ID lang flagvar src)
       AND MSVC_${lang}_ARCHITECTURE_ID)
     foreach(userflags "${CMAKE_${lang}_COMPILER_ID_FLAGS_LIST}" "")
       CMAKE_DETERMINE_MSVC_SHOWINCLUDES_PREFIX(${lang} "${userflags}")
+      if(CMAKE_${lang}_CL_SHOWINCLUDES_PREFIX)
+        break()
+      endif()
     endforeach()
   else()
     set(CMAKE_${lang}_CL_SHOWINCLUDES_PREFIX "")
@@ -1140,9 +1143,12 @@ function(CMAKE_DETERMINE_MSVC_SHOWINCLUDES_PREFIX lang userflags)
     RESULT_VARIABLE res
     ENCODING AUTO # cl prints in console output code page
     )
+  string(REPLACE "\n" "\n  " msg "  ${out}")
   if(res EQUAL 0 AND "${out}" MATCHES "(^|\n)([^:\n]*:[^:\n]*:[ \t]*)")
     set(CMAKE_${lang}_CL_SHOWINCLUDES_PREFIX "${CMAKE_MATCH_2}" PARENT_SCOPE)
+    string(APPEND msg "\nFound prefix \"${CMAKE_MATCH_2}\"")
   else()
     set(CMAKE_${lang}_CL_SHOWINCLUDES_PREFIX "" PARENT_SCOPE)
   endif()
+  message(CONFIGURE_LOG "Detecting ${lang} compiler /showIncludes prefix:\n${msg}\n")
 endfunction()