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/mempool.c

@@ -76,7 +76,7 @@ void mempool_free_tile(struct mempool *mp, void *p) {
         mp->freelist = p;
 }
 
-#ifdef VALGRIND
+#if VALGRIND
 
 void mempool_drop(struct mempool *mp) {
         struct pool *p = mp->first_pool;