This make it easier to manage separate compilation units (static functions and
global variables, local type definitions, etc.)
The generated header file `functions.h' caused a circular dependency problem;
it wasn't updated automatically when changes were made to the sources (e.g.
new function definition). The sources can't be in the dependency list of
`functions.h' in the Makefile, because `functions.h' is in the dependency list
of each source file. GNU make is able to ignore the circular dependency but not
BSD make.
At any rate, keeping the prototype list up-to-date is easy, because the
compiler will complain if a function is used in a compilation unit but defined
in an other one.
It also makes static analysis easier out of the box.
