VS: Compute project GUIDs deterministically

Compute deterministic GUIDs that are unique to the build tree by
hashing the path to the build tree with the GUID logical name.
Avoid storing them in the cache, but honor any found there.
This will allow project GUIDs to be reproduced in a fresh build
tree so long as its path is the same as the original, which may
be useful for incremental builds.

Source/cmLocalVisualStudio10Generator.cxx

@@ -107,10 +107,9 @@ void cmLocalVisualStudio10Generator
   cmVS10XMLParser parser;
   parser.ParseFile(path);
 
-  // if we can not find a GUID then create one
+  // if we can not find a GUID then we will generate one later
   if(parser.GUID.empty())
     {
-    this->GlobalGenerator->CreateGUID(name);
     return;
     }