locale.c: Make statics of repeated string constants

These strings are (or soon will be) used in multiple places; so have
just one definition for them.

locale.c

@@ -126,6 +126,11 @@ static int debug_initialization = 0;
  * the compiler warnings about comparing unlike signs */
 #define STRLENs(s)  (sizeof("" s "") - 1)
 
+/* Default values come from the C locale */
+static const char C_codeset[] = "ANSI_X3.4-1968";
+static const char C_decimal_point[] = ".";
+static const char C_thousands_sep[] = "";
+
 /* Is the C string input 'name' "C" or "POSIX"?  If so, and 'name' is the
  * return of setlocale(), then this is extremely likely to be the C or POSIX
  * locale.  However, the output of setlocale() is documented to be opaque, but
@@ -1501,7 +1506,7 @@ S_set_numeric_radix(pTHX_ const bool use_locale)
     const char * radix = (use_locale)
                          ? my_nl_langinfo(RADIXCHAR, FALSE)
                                         /* FALSE => already in dest locale */
-                         : ".";
+                         : C_decimal_point;
 
         sv_setpv(PL_numeric_radix_sv, radix);
 
@@ -1599,9 +1604,9 @@ S_new_numeric(pTHX_ const char *newnum)
      * THOUSEP can currently (but rarely) cause a race, so avoid doing that,
      * and just always change the locale if not C nor POSIX on those systems */
     if (! PL_numeric_standard) {
-        PL_numeric_standard = cBOOL(strEQ(".", my_nl_langinfo(RADIXCHAR,
+        PL_numeric_standard = cBOOL(strEQ(C_decimal_point, my_nl_langinfo(RADIXCHAR,
                                             FALSE /* Don't toggle locale */  ))
-                                 && strEQ("",  my_nl_langinfo(THOUSEP, FALSE)));
+                                 && strEQ(C_thousands_sep,  my_nl_langinfo(THOUSEP, FALSE)));
     }
 
 #    endif
@@ -3403,7 +3408,7 @@ S_my_nl_langinfo(const int item, bool toggle)
             const char * name = querylocale_c(LC_CTYPE);
 
             if (isNAME_C_OR_POSIX(name)) {
-                return "ANSI_X3.4-1968";
+                return C_codeset;
             }
 
             /* Find the dot in the locale name */
@@ -3746,7 +3751,7 @@ Perl_init_i18nl10n(pTHX_ int printwarn)
 #  endif
 #  ifdef USE_LOCALE_NUMERIC
 
-    PL_numeric_radix_sv = newSVpvs(".");
+    PL_numeric_radix_sv    = newSVpvn(C_decimal_point, strlen(C_decimal_point));
 
 #  endif
 #  ifdef LOCALE_ENVIRON_REQUIRED