[cmake] Use CMAKE_GENERATOR to determine if Ninja is used

[mcf]

Summary

The name of the make program does not necessarily match "ninja",
especially if an alternative implementation like samurai is used.

Using CMAKE_GENERATOR is a more robust detection method, and is
already used elsewhere in this file.

[mcf]

Thanks! I don't have commit access. Are you able to submit this change?

[mcf]

Ping. What do I need to do to submit this change?

[mcf]

Ping.

llvm/cmake/modules/HandleLLVMOptions.cmake

 set(LLVM_ENABLE_LTO OFF CACHE STRING "Build LLVM with LTO. May be specified as Thin or Full to use a particular kind of LTO")
 string(TOUPPER "${LLVM_ENABLE_LTO}" uppercase_LLVM_ENABLE_LTO)
 
 # Ninja Job Pool support
 # 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 (Ninja only).")
 if(LLVM_PARALLEL_COMPILE_JOBS)
-  if(NOT CMAKE_MAKE_PROGRAM MATCHES "ninja")
+  if(NOT CMAKE_GENERATOR STREQUAL "Ninja")
     message(WARNING "Job pooling is only available with Ninja generators.")
   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 (Ninja only).")
-if(CMAKE_MAKE_PROGRAM MATCHES "ninja")
+if(CMAKE_GENERATOR STREQUAL "Ninja")
   if(NOT LLVM_PARALLEL_LINK_JOBS AND uppercase_LLVM_ENABLE_LTO STREQUAL "THIN")
     message(STATUS "ThinLTO provides its own parallel linking - limiting parallel link jobs to 2.")
     set(LLVM_PARALLEL_LINK_JOBS "2")
   endif()
   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)
   endif()