Add FFI extension

[drigolin]

FFI is very useful when using some PHP libraries like vips.

[jingjingxyk]

FFI 扩展已经添加到实验分支，用实验分支自己测试。 https://github.com/swoole/swoole-cli/pull/704/files

The FFI extension has been added, please use the experimental branch to test it

git clone -b experiment https://github.com/swoole/swoole-cli.git

cd swoole-cli

php prepare.php +ffi

使用快速构建脚本
Use Quick Build Script

cp -f build-release-example.sh build-release-php.sh 

# edit this file
vi build-release-php.sh

OPTIONS="${OPTIONS} +ffi 

bash build-release-php.sh

[KingBes]

如果没有ffi，那么会想办法做一个平替版吗？还有静态编译后linux用不了ffi的情况