Treat config.pl as optional in vcregress.pl

This is how build.pl treats it and how it's documented.

Author: petere

src/tools/msvc/vcregress.pl

@@ -13,7 +13,8 @@
 
 chdir "../../.." if (-d "../../../src/tools/msvc");
 
-require 'src/tools/msvc/config.pl';
+require 'src/tools/msvc/config_default.pl';
+require 'src/tools/msvc/config.pl' if (-f 'src/tools/msvc/config.pl');
 
 # buildenv.pl is for specifying the build environment settings
 # it should contian lines like: