利用cmake来编译sim800c,sim868 app

sim原厂提供的编译方法是直接用makefile

它有个缺点是即使只改动一个文件，每次都要重新编译所有源文件

目前项目功能越做越多，源文件也比较多，每次编译需要花较长时间，有点浪费

采用cmake编译的好处是只编译修改过的文件，节省时间。

经测试效果良好，在Visual Studio中编译时很快了。

效果如下:

1>------ Build started: Project: simapp, Configuration: Debug Win32 ------
1>iotclient.cpp
1>simapp.vcxproj -> D:\bear.simapp\project\simapp\Debug\simapp.exe
1>config=-D_CONFIG_VS_OUTPUT=1
1>-- Configuring done
1>-- Generating done
1>-- Build files have been written to: D:/bear.simapp/project/simapp/bin.868
1>Scanning dependencies of target simapp
1>[  1%] Building CXX object CMakeFiles/simapp.dir/src/iotclient.o
1>[  3%] Linking CXX static library libsimapp.a
1>[100%] Built target simapp
1>[100%] Built target CustomLinkSimApp
1>-- Configuring done
1>-- Generating done
1>-- Build files have been written to: D:/bear.simapp/project/simapp/bin.800c
1>Scanning depend