A project building tool, or project management tool, allows you to automatically rebuild a program or library containing a large number of source files. Generally a project building tool will recompile only those files which have changed, or depend on files which have changed. The project building tool also insulates you from setting all the options for gcc and other programs.
The project building tool most often used with the GNU compiler suite is GNU make. I won't attempt to explain how to use make here. See the documentation for make from the Free Software Foundation for that.
One alternative to make is a tool called Jam, which is (in my experience) easier to use, but make is much more common.