Conditionally define CDECL and FASTCALL on windows

These may already be defined by other includes. Hopefully puts out the windows
fire.

Author: brson

src/rt/rust.h

@@ -9,8 +9,12 @@
 #ifdef __i386__
 // 'cdecl' ABI only means anything on i386
 #ifdef __WIN32__
+#ifndef CDECL
 #define CDECL __cdecl
+#endif
+#ifndef FASTCALL
 #define FASTCALL __fastcall
+#endif
 #else
 #define CDECL __attribute__((cdecl))
 #define FASTCALL __attribute__((fastcall))