-Wmissing-declarations enforces that every function (except for static
functions) must be declared separately before it's defined. This
essentially enforces that every function must be either static, or
declared in a header elsewhere.

This helps the optimizer, as it can do a better job of inlining if it
knows that a function won't be used outside of a given file. It also
helps -Wunused-function (which is enabled by -Wall) find more unused
functions.

Note that Clang spells this option -Wmissing-prototypes, which
confusingly is the name of a related but different warning option under
GCC.