Crank up the warning levels, and adjust the code to silence ...

several compilers:
- gcc version 4.5.3 (NetBSD nb2 20110806)
- gcc version 4.9.2 (Ubuntu 4.9.2-10ubuntu13)
- Ubuntu clang version 3.6.0-2ubuntu1 (tags/RELEASE_360/final) (based on LLVM 3.6.0)

The warnings were mostly about local variables shadowing others,
unused function parameters, and C++ style comments. Some variables were
indeed used before set.

Note that on Linux, using -std=c99 does stupid things like *remove* the
declaration of strdup() from <string.h>. Therefore I've reluctantly used
-std=gnu99.

extree.c

@@ -724,6 +724,9 @@ EX_TREE        *evaluate(
             res->data.child.right = right;
         }
         break;
+    default:
+        fprintf(stderr, "Invalid tree\n");
+        return NULL;
     }
 
     res->cp = cp;