add GCC build flags for largefile >2GB support

dtool/Config.Linux.pp

@@ -127,3 +127,7 @@
 // What is the syntax of the STL allocator declaration?  See
 // LocalSetup.pp for allowable values.
 #define STL_ALLOCATOR GNU
+
+// large file >2GB support
+#define _LARGEFILE_SOURCE 1
+#define _FILE_OFFSET_BITS 64