Support defined ONNX_ML in parent cmake files (onnx#1821)

Author: houseroad

CMakeLists.txt

@@ -24,12 +24,14 @@ option(ONNX_COVERAGE "Build with coverage instrumentation" OFF)
 option(ONNX_BUILD_TESTS "Build ONNX C++ APIs Tests" OFF)
 option(ONNX_USE_LITE_PROTO "Use lite protobuf instead of full." OFF)
 option(ONNXIFI_ENABLE_EXT "Enable onnxifi extensions." OFF)
-if(DEFINED ENV{ONNX_ML})
-  set(DEFAULT_ONNX_ML $ENV{ONNX_ML})
-else()
-  set(DEFAULT_ONNX_ML ON)
+if(NOT DEFINED ONNX_ML)
+  if(DEFINED ENV{ONNX_ML})
+    set(DEFAULT_ONNX_ML $ENV{ONNX_ML})
+  else()
+    set(DEFAULT_ONNX_ML ON)
+  endif()
+  option(ONNX_ML "Enable traditional ML API." ${DEFAULT_ONNX_ML})
 endif()
-option(ONNX_ML "Enable traditional ML API." ${DEFAULT_ONNX_ML})
 option(ONNXIFI_DUMMY_BACKEND "Use dummy backend in onnxifi test driver." OFF)
 
 # Set C++11 as standard for the whole project