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.

src/jemalloc_cpp.cpp

@@ -2,7 +2,6 @@
 #include <new>
 // NOLINTBEGIN(misc-use-anonymous-namespace)
 
-#define JEMALLOC_CPP_CPP_
 #ifdef __cplusplus
 extern "C" {
 #endif