Make headers self-contained by fixing #includes

Header files are now self-contained, which makes the relationships
between the files clearer, and crucially allows LSP tools like `clangd`
to function correctly in all of our header files. I have verified that
the headers are self-contained (aside from the various Windows shims) by
compiling them as if they were C files – in a follow-up commit I plan to
add this to CI to ensure we don't regress on this front.

test/integration/MALLOCX_ARENA.c

@@ -2,14 +2,6 @@
 
 #define NTHREADS 10
 
-static bool have_dss =
-#ifdef JEMALLOC_DSS
-    true
-#else
-    false
-#endif
-    ;
-
 void *
 thd_start(void *arg) {
 	unsigned thread_ind = (unsigned)(uintptr_t)arg;

test/integration/allocated.c

@@ -1,13 +1,5 @@
 #include "test/jemalloc_test.h"
 
-static const bool config_stats =
-#ifdef JEMALLOC_STATS
-    true
-#else
-    false
-#endif
-    ;
-
 void *
 thd_start(void *arg) {
 	int err;