for help on building a R package with several R function and a bunch of c, c++

Hope I can get quick help from here, I have a bunch of c, c++ included main function and makefile. It works well on both UNIX and windows. I tried to build R package which include this C++ program and several other R functions.  These R functions here are independent of c++ code. I prefer to define one R function to call this c++ program. 

I know how to build R package with just R function, But I don't know how to handle those c++, c and makefile  after I copied them in  /src.  I am reading manual "Writing R Extensions", I didn't catch the key point . Do you guys anybody know easy way to do it?.

Thank you in advance! 

Alex