reroute redefinition of NDEBUG

NDEBUG can be defined via a command-line argument.  Should take care of
such situations.

array.c

@@ -32,6 +32,7 @@
 #include "builtin.h"
 
 #if !ARRAY_DEBUG
+# undef NDEBUG
 # define NDEBUG
 #endif
 #include "ruby_assert.h"

complex.c

@@ -15,6 +15,7 @@
 #include <ctype.h>
 #include <math.h>
 
+#undef NDEBUG
 #define NDEBUG
 #include "id.h"
 #include "internal.h"

id_table.c

@@ -7,6 +7,7 @@
 #endif
 
 #if ID_TABLE_DEBUG == 0
+#undef NDEBUG
 #define NDEBUG
 #endif
 #include "ruby_assert.h"

rational.c

@@ -20,6 +20,7 @@
 #include <gmp.h>
 #endif
 
+#undef NDEBUG
 #define NDEBUG
 #include "id.h"
 #include "internal.h"