Add "3" suffix to default user config file

Index: xpdf/config.h
--- xpdf/config.h.orig
+++ xpdf/config.h
@@ -55,7 +55,7 @@
 #if defined(VMS) || defined(_WIN32)
 #define xpdfUserConfigFile "xpdfrc"
 #else
-#define xpdfUserConfigFile ".xpdfrc"
+#define xpdfUserConfigFile ".xpdfrc3"
 #endif
 
 // system config file name (set via the configure script)
