print llvm build choice and allow cmake arg to override env var (#2266)

* print llvm build choice and allow cmake arg to override env var

* fix cmake error

* use existing function

CMakeLists.txt

@@ -16,6 +16,9 @@
 
 cmake_minimum_required (VERSION 3.1)
 
+# set directory where the custom finders live
+set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules/")
+
 if("${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_BINARY_DIR}")
     message(FATAL_ERROR "In-source builds are not allowed.")
 endif()
@@ -24,7 +27,8 @@ if("${CMAKE_GENERATOR}" STREQUAL "Ninja")
     message(FATAL_ERROR "Ninja generator not supported.")
 endif()
 
-include(cmake/Modules/git_tags.cmake)
+include(git_tags)
+include(var_functions)
 
 NGRAPH_GET_VERSION_LABEL()
 
@@ -88,18 +92,11 @@ message(STATUS "NGRAPH_FORWARD_CMAKE_ARGS ${NGRAPH_FORWARD_CMAKE_ARGS}")
 # Prevent Eigen from using any LGPL3 code
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DEIGEN_MPL2_ONLY -DTBB_USE_THREADING_TOOLS")
 
-if($ENV{NGRAPH_USE_PREBUILT_LLVM})
-    set(NGRAPH_USE_PREBUILT_LLVM TRUE)
-endif()
+ngraph_var(NGRAPH_USE_PREBUILT_LLVM DEFAULT "FALSE")
 
 # Create compilation database compile_commands.json
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
-# set directory where the custom finders live
-set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules/")
-
-include(var_functions)
-
 option(NGRAPH_UNIT_TEST_ENABLE "Control the building of unit tests" TRUE)
 option(NGRAPH_TOOLS_ENABLE "Control the building of tool" TRUE)
 option(NGRAPH_CPU_ENABLE "Control the building of the CPU backend" TRUE)
@@ -135,6 +132,7 @@ message(STATUS "NGRAPH_DEX_ONLY:              ${NGRAPH_DEX_ONLY}")
 message(STATUS "NGRAPH_CODE_COVERAGE_ENABLE:  ${NGRAPH_CODE_COVERAGE_ENABLE}")
 message(STATUS "NGRAPH_LIB_VERSIONING_ENABLE: ${NGRAPH_LIB_VERSIONING_ENABLE}")
 message(STATUS "NGRAPH_PYTHON_BUILD_ENABLE:   ${NGRAPH_PYTHON_BUILD_ENABLE}")
+message(STATUS "NGRAPH_USE_PREBUILT_LLVM:     ${NGRAPH_USE_PREBUILT_LLVM}")
 
 if (NGRAPH_ONNX_IMPORT_ENABLE)
     option(NGRAPH_USE_SYSTEM_PROTOBUF "Use system provided Protobuf shared object" FALSE)