[Andrew-Dufresne]

Besides subsystems, tools like make and nmake utility can also reduce the compile time.

Make figures out automatically which files it needs to update, based on which source files have changed. It also automatically determines the proper order for updating files, in case one non-source file depends on another non-source file. As a result, if you change a few source files and then run Make, it does not need to recompile all of your program. It updates only those non-source files that depend directly or indirectly on the source files that you changed.

[voxcogitatio]

Yes, but large projects often need to use recursive makefiles, which are notoriously flaky at detecting what has changed (if anything).

[drothlis]

No project needs to use recursive make.

"Recursive make considered harmful": http://miller.emu.id.au/pmiller/books/rmch/