Better sanity check when making base directory.

This should catch idiot coders (i.e. me) who call getSubdirPath('build')
instead of the correct getSubdirPath('build-dir').

kdesrc-build

@@ -1597,6 +1597,11 @@ HOME
         # If build-dir starts with a slash, it is an absolute path.
         return $dir if $dir =~ /^\//;
 
+        # Make sure we got a valid option result.
+        if (!$dir) {
+            confess ("Reading option for $subdirOption gave empty \$dir!");
+        }
+
         # If it starts with a tilde, expand it out.
         if ($dir =~ /^~/)
         {