adding 64bit flags

dtool/Config.Linux.pp

@@ -22,7 +22,10 @@
 #define IS_LINUX 1
 
 // What additional flags should we pass to interrogate?
-#define SYSTEM_IGATE_FLAGS -D__i386__ -D__const=const -Dvolatile -Dmutable
+#if $[eq $[shell uname -i], x86_64] // if Linux is 64bit
+  #define SYSTEM_IGATE_FLAGS -D_LP64
+#else
+  #define SYSTEM_IGATE_FLAGS -D__i386__ -D__const=const -Dvolatile -Dmutable
 
 // Is the platform big-endian (like an SGI workstation) or
 // little-endian (like a PC)?  Define this to the empty string to