py: Make it easy to build without MICROPY_PY_BUILTINS_COMPLEX.

Automagically skip related modules.
diff --git a/py/modcmath.c b/py/modcmath.c
index 570341d..33fe9c7 100644
--- a/py/modcmath.c
+++ b/py/modcmath.c
@@ -26,7 +26,7 @@
 
 #include "py/builtin.h"
 
-#if MICROPY_PY_BUILTINS_FLOAT && MICROPY_PY_CMATH
+#if MICROPY_PY_BUILTINS_FLOAT && MICROPY_PY_BUILTINS_COMPLEX && MICROPY_PY_CMATH
 
 #include <math.h>