Lets consider your /usr/local/lib/libYARP_OS.a. What you can do is, have -L/usr/local/lib/ in your makefile as one of the variables. And then you can have -lYARP_SO appended to the LDLIBS.

-L is for path to the lib and -l is the lib name here libYARP_OS.a will be passed as -lYARP_OS. On command line you would do something like: gcc -o main main.c -L/usr/local/lib/ -lYARP_SO. This should give you an idea.