[CMake] Add warning for using compile and link pools on unsupported versions of CMake or non-ninja generators.

[beanz]

Summary

I've gotten feedback from users on CMake 2.8 that the compile and link pool options were not working. This is expected so I'm adding a warning so we can report invalid configurations to users.

[eugenis]

LGTM, thanks

llvm/trunk/CMakeLists.txt

 endif()
 
 project(LLVM)
 
 # The following only works with the Ninja generator in CMake >= 3.0.
 set(LLVM_PARALLEL_COMPILE_JOBS "" CACHE STRING
   "Define the maximum number of concurrent compilation jobs.")
 if(LLVM_PARALLEL_COMPILE_JOBS)
+  if(CMAKE_VERSION VERSION_LESS 3.0 OR NOT CMAKE_MAKE_PROGRAM STREQUAL "ninja")
+    message(WARNING "Job pooling is only available with Ninja generators and CMake 3.0 and later.")
+  else()
     set_property(GLOBAL APPEND PROPERTY JOB_POOLS compile_job_pool=${LLVM_PARALLEL_COMPILE_JOBS})
     set(CMAKE_JOB_POOL_COMPILE compile_job_pool)
   endif()
+endif()
 
 set(LLVM_PARALLEL_LINK_JOBS "" CACHE STRING
   "Define the maximum number of concurrent link jobs.")
 if(LLVM_PARALLEL_LINK_JOBS)
+  if(CMAKE_VERSION VERSION_LESS 3.0 OR NOT CMAKE_MAKE_PROGRAM STREQUAL "ninja")
+    message(WARNING "Job pooling is only available with Ninja generators and CMake 3.0 and later.")
+  else()
     set_property(GLOBAL APPEND PROPERTY JOB_POOLS link_job_pool=${LLVM_PARALLEL_LINK_JOBS})
     set(CMAKE_JOB_POOL_LINK link_job_pool)
   endif()
+endif()
 
 # Add path for custom modules
 set(CMAKE_MODULE_PATH
   ${CMAKE_MODULE_PATH}
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules"
   )