Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -46,7 +46,9 @@
     set(LLVM_CMAKE_PATH "${LLVM_BINARY_DIR}/share/llvm/cmake")
     set(LLVM_LIT_PATH "${LLVM_PATH}/utils/lit/lit.py")
   else()
-    message(FATAL_ERROR "llvm-config not found and LLVM_MAIN_SRC_DIR not defined")
+    message(FATAL_ERROR "llvm-config not found and LLVM_MAIN_SRC_DIR not defined. "
+                        "Reconfigure with -DLLVM_CONFIG=path/to/llvm-config "
+                        "or -DLLVM_PATH=path/to/llvm-source-root.")
   endif()
 
   if(EXISTS ${LLVM_CMAKE_PATH})
Index: www/index.html
===================================================================
--- www/index.html
+++ www/index.html
@@ -96,6 +96,16 @@
     linux you may need -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++</code></li>
   <li><code>make</code></li>
   </ul>
+  <p> By default CMake uses <code>llvm-config</code> to locate the required
+      LLVM sources. If CMake cannot find <code>llvm-config</code> then you must
+      configure CMake using either of the following options.
+  </p>
+  <ul>
+  <li><code>-DLLVM_CONFIG=path/to/llvm-config</code></li>
+  <li><code>-DLLVM_PATH=path/to/llvm-source-root</code></li>
+  </ul>
+
+  </p>
 
   <p>To run the tests:</p>
   <ul>