Allow building with MSVC and Strawberry perl

Strawberry uses __builtin_expect which Visual C doesn't have. For this
case define it as a noop. Solution taken from vim sources.

Backpatch to all live branches

src/pl/plperl/plperl.h

@@ -54,6 +54,8 @@
 #ifdef isnan
 #undef isnan
 #endif
+/* Work around for using MSVC and Strawberry Perl >= 5.30. */
+#define __builtin_expect(expr, val) (expr)
 #endif
 
 /*