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/libsystemd-network/test-dhcp-client.c

@@ -539,7 +539,7 @@ int main(int argc, char *argv[]) {
         test_discover_message(e);
         test_addr_acq(e);
 
-#ifdef VALGRIND
+#if VALGRIND
         /* Make sure the async_close thread has finished.
          * valgrind would report some of the phread_* structures
          * as not cleaned up properly. */