Rename bultins config variables to MICROPY_PY_BUILTINS_*.

This renames:
MICROPY_PY_FROZENSET -> MICROPY_PY_BUILTINS_FROZENSET
MICROPY_PY_PROPERTY -> MICROPY_PY_BUILTINS_PROPERTY
MICROPY_PY_SLICE -> MICROPY_PY_BUILTINS_SLICE
MICROPY_ENABLE_FLOAT -> MICROPY_PY_BUILTINS_FLOAT

See issue #35 for discussion.
diff --git a/py/objcomplex.c b/py/objcomplex.c
index 80a13c6..d58b534 100644
--- a/py/objcomplex.c
+++ b/py/objcomplex.c
@@ -36,7 +36,7 @@
 #include "runtime0.h"
 #include "runtime.h"
 
-#if MICROPY_ENABLE_FLOAT
+#if MICROPY_PY_BUILTINS_FLOAT
 
 #include <math.h>