Print an error message if the lib directory (first argument) is not a
directory.

llvm-svn: 29508
diff --git a/llvm/utils/GenLibDeps.pl b/llvm/utils/GenLibDeps.pl
index 9bb4752..419d8f9 100755
--- a/llvm/utils/GenLibDeps.pl
+++ b/llvm/utils/GenLibDeps.pl
@@ -24,9 +24,10 @@
 
 # Give first option a name.
 my $Directory = $ARGV[0];
-if (!defined($Directory)) {
-  die "First argument must be the directory containing LLVM libs\n";
+if (!defined($Directory) || ! -d "$Directory") {
+  die "First argument must specify the directory containing LLVM libs\n";
 }
+
 my $nmPath = $ARGV[1];
 
 # Find the "dot" program