Fix standalone build (#415) (#491)

Fixes #406 Fixes
#374

(cherry picked from commit 529a918)

Co-authored-by: František Zatloukal <[email protected]>

Author: wenju-he

cl_headers/CMakeLists.txt

@@ -1,5 +1,9 @@
 set(CL_HEADERS_LIB cl_headers)
-set(CLANG_COMMAND $<TARGET_FILE:clang> )
+if(USE_PREBUILT_LLVM)
+    find_program(CLANG_COMMAND clang PATHS ${LLVM_TOOLS_BINARY_DIR} NO_DEFAULT_PATH)
+else()
+    set(CLANG_COMMAND $<TARGET_FILE:clang>)
+endif()
 if(LLVM_USE_HOST_TOOLS AND NOT OPENCL_CLANG_BUILD_EXTERNAL)
   build_native_tool(clang CLANG_COMMAND)
 endif()