only check for prebuilt llvm and API flags if llvm is to be used in build (#2089)

CMakeLists.txt

@@ -74,10 +74,6 @@ if($ENV{NGRAPH_USE_PREBUILT_LLVM})
     set(NGRAPH_USE_PREBUILT_LLVM TRUE)
 endif()
 
-if (DEFINED NGRAPH_USE_CXX_ABI AND NGRAPH_USE_PREBUILT_LLVM)
-    message(FATAL_ERROR "Unable to use NGRAPH_USE_PREBUILT_LLVM with NGRAPH_USE_CXX_ABI")
-endif()
-
 # These variables are undocumented but useful.
 set(CMAKE_DISABLE_SOURCE_CHANGES ON)
 set(CMAKE_DISABLE_IN_SOURCE_BUILD ON)

src/ngraph/codegen/CMakeLists.txt

@@ -16,6 +16,10 @@
 
 
 if (NGRAPH_GPU_ENABLE OR (NGRAPH_CPU_ENABLE AND NOT NGRAPH_DEX_ONLY))
+    if (DEFINED NGRAPH_USE_CXX_ABI AND NGRAPH_USE_PREBUILT_LLVM)
+        message(FATAL_ERROR "Unable to use NGRAPH_USE_PREBUILT_LLVM with NGRAPH_USE_CXX_ABI")
+    endif()
+
     set(SRC
         code_writer.cpp
         compiler.cpp