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.

scripts/run_static_analysis.sh

@@ -12,8 +12,16 @@ compile_time_malloc_conf='background_thread:true,'\
 'zero_realloc:free,'\
 'prof_unbias:false,'\
 'prof_time_resolution:high'
+extra_flags=(
+	-Wmissing-prototypes
+	-Wmissing-variable-declarations
+	-Wstrict-prototypes
+	-Wunreachable-code
+	-Wunreachable-code-aggressive
+	-Wunused-macros
+)
 
-EXTRA_CFLAGS='-Wstrict-prototypes' EXTRA_CXXFLAGS='-Wstrict-prototypes' ./autogen.sh \
+EXTRA_CFLAGS="${extra_flags[*]}" EXTRA_CXXFLAGS="${extra_flags[*]}" ./autogen.sh \
 	--with-private-namespace=jemalloc_ \
 	--disable-cache-oblivious \
 	--enable-prof \