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

[Chris Bieneman]

Hi eugenis,

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.

http://reviews.llvm.org/D10193

Files:
  CMakeLists.txt

Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -32,15 +32,23 @@
 set(LLVM_PARALLEL_COMPILE_JOBS "" CACHE STRING
   "Define the maximum number of concurrent compilation jobs.")
 if(LLVM_PARALLEL_COMPILE_JOBS)
-  set_property(GLOBAL APPEND PROPERTY JOB_POOLS compile_job_pool=${LLVM_PARALLEL_COMPILE_JOBS})
-  set(CMAKE_JOB_POOL_COMPILE compile_job_pool)
+  if(CMAKE_VERSION VERSION_LESS 3.0 OR NOT CMAKE_MAKE_PROGRAM STREQUAL "ninja")
+    message(WARNING "Job pooling is only available with Ninja generators and CMake 3.0 and later.")
+  else()
+    set_property(GLOBAL APPEND PROPERTY JOB_POOLS compile_job_pool=${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)
-  set_property(GLOBAL APPEND PROPERTY JOB_POOLS link_job_pool=${LLVM_PARALLEL_LINK_JOBS})
-  set(CMAKE_JOB_POOL_LINK link_job_pool)
+  if(CMAKE_VERSION VERSION_LESS 3.0 OR NOT CMAKE_MAKE_PROGRAM STREQUAL "ninja")
+    message(WARNING "Job pooling is only available with Ninja generators and CMake 3.0 and later.")
+  else()
+    set_property(GLOBAL APPEND PROPERTY JOB_POOLS link_job_pool=${LLVM_PARALLEL_LINK_JOBS})
+    set(CMAKE_JOB_POOL_LINK link_job_pool)
+  endif()
 endif()
 
 # Add path for custom modules

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