fix errtbl.c to compile under gcc

When errtbl.c is being compiled for the errcode executable,
(when ERRCODE_SRC is defined), skip functions that make
called to math_error() and name_newerrorstr() from libcalc.

errtbl.c

@@ -1683,6 +1683,12 @@ errsym_2_errnum(CONST char *errsym)
 }
 
 
+/*
+ * The code below is not compatible with the errcode executable
+ */
+#if !defined(ERRCODE_SRC)
+
+
 /*
  * errnum_2_errsym - convert an errnum value into an errsym E_STRING, possibly malloced
  *
@@ -1934,6 +1940,9 @@ errsym_2_errmsg(CONST char *errsym, bool *palloced)
 }
 
 
+#endif /* !ERRCODE_SRC */
+
+
 /*
  * The code below is used to form the errcode executable and is NOT part of libcalc
  */