builtin: skip alloca.h include on OpenBSD (#81)

alloca() is provided by stdlib.h on OpenBSD, so the separate alloca.h
include is not needed and causes a faild build.

Author: jturner

internal/compiler/builtin/builtin.h

@@ -4,7 +4,7 @@
 
 #ifdef _WIN32
 #include <malloc.h>
-#else
+#elif !defined(__OpenBSD__)
 #include <alloca.h>
 #endif