Enable a few additional warnings for CI and fix the issues they uncovered

- `-Wmissing-prototypes` and `-Wmissing-variable-declarations` are
  helpful for finding dead code and/or things that should be `static`
  but aren't marked as such.
- `-Wunused-macros` is of similar utility, but for identifying dead macros.
- `-Wunreachable-code` and `-Wunreachable-code-aggressive` do exactly
  what they say: flag unreachable code.

include/jemalloc/jemalloc_protos.h.in

@@ -4,6 +4,7 @@
  * JEMALLOC_NO_DEMANGLE is defined (see jemalloc_mangle@install_suffix@.h).
  */
 extern JEMALLOC_EXPORT const char	*@je_@malloc_conf;
+extern JEMALLOC_EXPORT const char	*@je_@malloc_conf_2_conf_harder;
 extern JEMALLOC_EXPORT void		(*@je_@malloc_message)(void *cbopaque,
     const char *s);