Check if FFI_GO_CLOSURES is defined (#796) This macro is always defined to 1 if defined, or undefined. With `-Wundef` option, checking the value without checking if it is defined causes warnings: ``` /opt/local/include/ffi.h:477:5: warning: 'FFI_GO_CLOSURES' is not defined, evaluates to 0 [-Wundef] #if FFI_GO_CLOSURES ^ ```