Using llvm-spirv as external llvm project

Author: AlexeySotkin

CMakeLists.txt

@@ -83,7 +83,11 @@ if(NOT USE_PREBUILT_LLVM)
     set(CLANG_SOURCE_DIR ${LLVM_SOURCE_DIR}/tools/clang)
     set(CLANG_BASE_REVISION master)
 
-    set(SPIRV_SOURCE_DIR ${LLVM_SOURCE_DIR}/projects/llvm-spirv)
+    if (LLVM_EXTERNAL_LLVM_SPIRV_SOURCE_DIR)
+        set(SPIRV_SOURCE_DIR ${LLVM_EXTERNAL_LLVM_SPIRV_SOURCE_DIR})
+    else (LLVM_EXTERNAL_LLVM_SPIRV_SOURCE_DIR)
+        set(SPIRV_SOURCE_DIR ${LLVM_SOURCE_DIR}/projects/llvm-spirv)
+    endif (LLVM_EXTERNAL_LLVM_SPIRV_SOURCE_DIR)
     set(SPIRV_BASE_REVISION master)
 
     apply_patches(${CLANG_SOURCE_DIR}