If you have an include file in a non-standard path, e.g. /opt/myincludes, you can include them with the -I option to gcc:
gcc -I/opt/myincludes -c cp.c -o cp.o
If you have includes from multiple directories, you can use more than one -I option:
gcc -I/opt/myincludes -I/opt/yourincludes ...
Note that when linking, you may need to link with libraries which are not in the default library path. You can include these library paths with the -L option. e.g. if you have a library file you need to link to: "/opt/mylibs/libmine.so", you would do it like this:
gcc -L/opt/mylibs -lmine cp.o -o cp