work out the cpu architecture at compile time, so MacOS fat binaries …

…work

ext/Platform.c

@@ -7,6 +7,25 @@
 
 static VALUE modulePlatform = Qnil;
 
+/*
+ * Determine the cpu type at compile time - useful for MacOSX where the the
+ * system installed ruby incorrectly reports 'host_cpu' as 'powerpc' when running
+ * on intel.
+ */
+#ifdef __i386__
+#define CPU "i386"
+#elif defined(__ppc__) || defined(__powerpc__)
+#define CPU "powerpc"
+#elif defined(__x86_64__)
+#define CPU "x86_64"
+#elif defined(__sparc__)
+#define CPU "sparc"
+#elif defined(__sparcv9__)
+#define CPU "sparcv9"
+#else
+#error "Unknown cpu type"
+#endif
+
 void
 rb_FFI_Platform_Init()
 {
@@ -18,6 +37,6 @@ rb_FFI_Platform_Init()
     rb_define_const(platform, "LITTLE_ENDIAN", INT2FIX(LITTLE_ENDIAN));
     rb_define_const(platform, "BIG_ENDIAN", INT2FIX(BIG_ENDIAN));
     rb_define_const(platform, "OS_", rb_str_new2(OS));
-    rb_define_const(platform, "ARCH_", rb_str_new2(ARCH));
+    rb_define_const(platform, "ARCH_", rb_str_new2(CPU));
     modulePlatform = platform;
 }