Add -Wmissing-prototypes to the build

With GCC 8.3.0 and Meson 0.53, I am not getting the warnings from -Wmissing-prototypes even with warning_level=3. We used to have this warning with autotools, and the code base has regressed since.

Fix all the warnings that crept into the code and reinstate -Wmissing-prototypes. The default fix is to make the functions static.