Support llvm headers in ${LLVM_LIBRARY_DIRS}/clang/${LLVM_VERSION_MAJOR}/ (#419)

Fedora places llvm include files into that directory instead of assumed
${LLVM_LIBRARY_DIRS}/clang/${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}/include/

Try both, fail horribly when none of them exist

Author: frantisekz

cl_headers/CMakeLists.txt

@@ -19,8 +19,15 @@ 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/")
+    if(EXISTS "${LLVM_LIBRARY_DIRS}/clang/${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}/")
+        set(OPENCL_HEADERS_DIR
+        "${LLVM_LIBRARY_DIRS}/clang/${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}/include/")
+    elseif(EXISTS "${LLVM_LIBRARY_DIRS}/clang/${LLVM_VERSION_MAJOR}/")
+        set(OPENCL_HEADERS_DIR
+            "${LLVM_LIBRARY_DIRS}/clang/${LLVM_VERSION_MAJOR}/include/")
+    else()
+        message(FATAL_ERROR "[OPENCL-CLANG] Couldn't find prebuilt LLVM include directory.")
+    endif()
 else(USE_PREBUILT_LLVM)
     set(OPENCL_HEADERS_DIR "${CLANG_SOURCE_DIR}/lib/Headers")
 endif(USE_PREBUILT_LLVM)