Ninja: add CMAKE_JOB_POOLS variable as default for JOBS_POOLS

This enables configuration of build parallelism with the Ninja generator by
passing arguments on the command line to CMake. For example,

  cmake \
    '-DCMAKE_JOB_POOLS:STRING=compile=5;link=2' \
    -DCMAKE_JOB_POOL_COMPILE:STRING=compile \
    -DCMAKE_JOB_POOL_LINK:STRING=link \
      ~/src/MyProject

Help/manual/cmake-variables.7.rst

@@ -50,6 +50,7 @@ Variables that Provide Information
    /variable/CMAKE_IMPORT_LIBRARY_SUFFIX
    /variable/CMAKE_JOB_POOL_COMPILE
    /variable/CMAKE_JOB_POOL_LINK
+   /variable/CMAKE_JOB_POOLS
    /variable/CMAKE_LANG_COMPILER_AR
    /variable/CMAKE_LANG_COMPILER_RANLIB
    /variable/CMAKE_LINK_LIBRARY_SUFFIX

Help/prop_gbl/JOB_POOLS.rst

@@ -19,5 +19,8 @@ Defined pools could be used globally by setting
 or per target by setting the target properties
 :prop_tgt:`JOB_POOL_COMPILE` and :prop_tgt:`JOB_POOL_LINK`.
 
+If not set, this property uses the value of the :variable:`CMAKE_JOB_POOLS`
+variable.
+
 Build targets provided by CMake that are meant for individual interactive
 use, such as ``install``, are placed in the ``console`` pool automatically.

Help/release/dev/cmake-job-pool.rst

@@ -0,0 +1,7 @@
+cmake-job-pool
+--------------
+
+* A :variable:`CMAKE_JOB_POOLS` variable was added specify a value to use for
+  the :prop_gbl:`JOB_POOLS` property. This enables control over build
+  parallelism with command line configuration parameters when using the Ninja
+  generator.

Help/variable/CMAKE_JOB_POOLS.rst

@@ -0,0 +1,6 @@
+CMAKE_JOB_POOLS
+---------------
+
+If the :prop_gbl:`JOB_POOLS` global property is not set, the value
+of this variable is used in its place.  See :prop_gbl:`JOB_POOLS`
+for additional information.

Source/cmLocalNinjaGenerator.cxx

@@ -207,6 +207,9 @@ void cmLocalNinjaGenerator::WritePools(std::ostream& os)
 
   const char* jobpools =
     this->GetCMakeInstance()->GetState()->GetGlobalProperty("JOB_POOLS");
+  if (!jobpools) {
+    jobpools = this->GetMakefile()->GetDefinition("CMAKE_JOB_POOLS");
+  }
   if (jobpools) {
     cmGlobalNinjaGenerator::WriteComment(
       os, "Pools defined by global property JOB_POOLS");