Library functions and objects that have been referenced will be automatically linked into your program, provided the library file is part of your project. The use of a function from one library file will not include any other functions from that library.
Your program will require declarations for any library functions or symbols it uses. Standard libraries come with standard C headers (.h files), which can be included into your source files. See your favorite C text book or Library Functions for the header that corresponds to each library function. Typically you would write library headers if you create your own library files.
Header files are not library files. Library files contain precompiled code, typically functions and variable definitions; header files provide declarations (as opposed to definitions) for those functions, variables and types in the library. Headers can also define preprocessor macros.