Check _M_X64 (Visual Studio, Windows/x86-64) in littleendian test.
Thanks to Norfanin for the tip!
--- a/src/physfs_internal.h Sat Sep 24 00:55:15 2011 -0700
+++ b/src/physfs_internal.h Wed Oct 05 03:58:52 2011 -0400
@@ -876,7 +876,8 @@
#define PHYSFS_LIL_ENDIAN 1234
#define PHYSFS_BIG_ENDIAN 4321
-#if defined(__i386__) || defined(__ia64__) || defined(_M_IX86) || defined(_M_IA64) || \
+#if defined(__i386__) || defined(__ia64__) || \
+ defined(_M_IX86) || defined(_M_IA64) || defined(_M_X64) \
(defined(__alpha__) || defined(__alpha)) || \
defined(__arm__) || defined(ARM) || \
(defined(__mips__) && defined(__MIPSEL__)) || \