Enable MRB_METHOD_T_STRUCT by default on 32bit Windows

Because we can't use the highest 2 bits of function pointers.

appveyor.yml

@@ -14,10 +14,18 @@ environment:
     # Visual Studio 2017 64bit
     - visualcpp: C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvars64.bat
 
+    # Visual Studio 2017 32bit
+    - visualcpp: C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvarsall.bat
+      machine: x86
+
     # Visual Studio 2015 64bit
     - visualcpp: C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat
       machine: amd64
 
+    # Visual Studio 2015 32bit
+    - visualcpp: C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat
+      machine: x86
+
     # Visual Studio 2013 64bit
     - visualcpp: C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\vcvarsall.bat
       machine: amd64

include/mrbconf.h

@@ -38,7 +38,12 @@
 
 /* add -DMRB_METHOD_T_STRUCT on machines that use higher bits of pointers */
 /* no MRB_METHOD_T_STRUCT requires highest 2 bits of function pointers to be zero */
-//#define MRB_METHOD_T_STRUCT
+#ifndef MRB_METHOD_T_STRUCT
+  // can't use highest 2 bits of function pointers on 32bit Windows.
+# if defined(_WIN32) && !defined(_WIN64)
+#   define MRB_METHOD_T_STRUCT
+# endif
+#endif
 
 /* add -DMRB_INT32 to use 32bit integer for mrb_int; conflict with MRB_INT64;
    Default for 32-bit CPU mode. */