Turn VALGRIND variable into a meson configuration switch

Configuration through environment variable is inconvenient with meson, because
they cannot be convieniently changed and/or are not preserved during
reconfiguration (https://github.com/mesonbuild/meson/issues/1503).
This adds -Dvalgrind=true/false, which has the advantage that it can be set
at any time with meson configure -Dvalgrind=... and ninja will rebuild targets
as necessary. Additional minor advantages are better consistency with the
options for hashmap debugging, and typo avoidance with '#if' instead of '#ifdef'.

src/basic/hashmap.c

@@ -281,7 +281,7 @@ static const struct hashmap_type_info hashmap_type_info[_HASHMAP_TYPE_MAX] = {
         },
 };
 
-#ifdef VALGRIND
+#if VALGRIND
 __attribute__((destructor)) static void cleanup_pools(void) {
         _cleanup_free_ char *t = NULL;
         int r;