[libcxxabi] Document the standalone build's requirements on llvm-config and llvm sources.

[EricWF]

Summary

The current documentation does not explain that the standalone build requires the LLVM sources. This patch updates the documentation to reflect this requirement and explains how to manually specify the location of the required files.

Any suggestions on better wording are encouraged.

[jroelofs]

www/index.html

100

perhaps you can also coerce cmake into barfing in such a way as to display this message when the appropriate circumstance comes up?

104

this one looks ok

105

but ones like this tend to confuse me when I see them because I'm never quite sure which folder it's asking me to point it at.

Some alternative bikeshed colors (none of which I particularly like):
<li><code>-DLLVM_PATH=path/to/llvm/</code></li>
<li><code>-DLLVM_PATH=path/to/llvm/lib/../</code></li>
<li><code>-DLLVM_PATH=path/to/llvm-source-root/</li>

[EricWF]

Implement jroelofs's suggestions. I now use -DLLVM_SOURCE=path/to/llvm-source-root. The error message produced my CMake is also more informative.

[EricWF]

Woops, the last patch uses LLVM_SOURCE instead of LLVM_PATH. Sorry for the spam.

[jroelofs]

Looks reasonable to me.

[EricWF]

Thanks. Committed as r220049.

CMakeLists.txt

     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})

www/index.html

     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>