Rust交叉编译paho-mqtt

[dependencies]
paho-mqtt = "0.11"
futures = "0.3"
env_logger = "0.9"

如果没有cmake，需要先安装cmake
sudo apt install cmake

该库依赖OpenSSL，需要先交叉编译OpenSSL。
若仍然找不到OpenSSL，执行命令，将OpenSSL目录加入到环境变量：
export OPENSSL_DIR=/home/nuoiot/tools/gcc-linaro-4.9.4-2017.01-x86_64_arm-linux-gnueabihf/arm-linux-gnueabihf
执行cargo交叉命令
cargo build --target=armv7-unknown-linux-gnueabihf --release

如果是x86或者x64:

export OPENSSL_INCLUDE_DIR=/usr/include/openssl 
export OPENSSL_LIB_DIR=/usr/lib/x86_64-linux-gnu