Fixes so that _ctypes can be compiled with the MingW compiler.

It seems that the definition of '__attribute__(x)' was responsible for
the compiler ignoring the '__fastcall' attribute on the
ffi_closure_SYSV function in libffi_msvc/ffi.c, took me quite some
time to figure this out.

Modules/_ctypes/libffi_msvc/fficonfig.h

@@ -3,8 +3,10 @@
 /* fficonfig.h.  Generated automatically by configure.  */
 /* fficonfig.h.in.  Generated automatically from configure.in by autoheader.  */
 
-/* Defines for MSVC */
+/* Define this for MSVC, but not for mingw32! */
+#ifdef _MSC_VER
 #define __attribute__(x) /* */
+#endif
 #define alloca _alloca
 
 /*----------------------------------------------------------------*/

Modules/_ctypes/libffi_msvc/ffitarget.h

@@ -71,11 +71,7 @@ typedef enum ffi_abi {
 #define FFI_TRAMPOLINE_SIZE 24
 #define FFI_NATIVE_RAW_API 0
 #else
-#ifdef _MSC_VER
-# define FFI_TRAMPOLINE_SIZE 15
-#else
-# define FFI_TRAMPOLINE_SIZE 10
-#endif
+#define FFI_TRAMPOLINE_SIZE 15
 #define FFI_NATIVE_RAW_API 1	/* x86 has native raw api support */
 #endif