Factor and simplify Makefile's and mpconfig, part 2.
diff --git a/py/showbc.c b/py/showbc.c
index eb7d41b..aea0aff 100644
--- a/py/showbc.c
+++ b/py/showbc.c
@@ -8,6 +8,8 @@
 #include "mpconfig.h"
 #include "bc0.h"
 
+#if MICROPY_SHOW_BC
+
 #define DECODE_UINT do { unum = *ip++; if (unum > 127) { unum = ((unum & 0x3f) << 8) | (*ip++); } } while (0)
 #define DECODE_ULABEL do { unum = (ip[0] | (ip[1] << 8)); ip += 2; } while (0)
 #define DECODE_SLABEL do { unum = (ip[0] | (ip[1] << 8)) - 0x8000; ip += 2; } while (0)
@@ -363,3 +365,5 @@
         printf("\n");
     }
 }
+
+#endif // MICROPY_SHOW_BC