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