diff --git a/cl_headers/CMakeLists.txt b/cl_headers/CMakeLists.txt
index 3dd2ea4..aeae6e9 100644
--- a/cl_headers/CMakeLists.txt
+++ b/cl_headers/CMakeLists.txt
@@ -11,12 +11,14 @@ add_custom_command(
 )
 endfunction(copy_file)
 
-if(USE_PREBUILT_LLVM)
-    set(OPENCL_HEADERS_DIR
-      "${LLVM_LIBRARY_DIRS}/clang/${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}/include/")
-else(USE_PREBUILT_LLVM)
-    set(OPENCL_HEADERS_DIR "${CLANG_SOURCE_DIR}/lib/Headers")
-endif(USE_PREBUILT_LLVM)
+if(NOT DEFINED OPENCL_HEADERS_DIR)
+  if(USE_PREBUILT_LLVM)
+      set(OPENCL_HEADERS_DIR
+        "${LLVM_LIBRARY_DIRS}/clang/${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}/include/")
+  else(USE_PREBUILT_LLVM)
+      set(OPENCL_HEADERS_DIR "${CLANG_SOURCE_DIR}/lib/Headers")
+  endif(USE_PREBUILT_LLVM)
+endif()
 copy_file(${OPENCL_HEADERS_DIR}/opencl-c.h opencl-c.h)
 
 add_custom_target (