Handle newlib C library's langinfo constant names

Handle newlib C library's langinfo constant names

The newlib C library (used in Cygwin) has alternative names for
nl_langinfo GNU extensions

* configure.ac (_NL_NUMERIC_GROUPING): new test
* libguile/i18n.c (INT_CURR_SYMBOL, MON_DECIMAL_POINT, MON_THOUSANDS_SEP)
   (MON_GROUPING, POSITIVE_SIGN, NEGATIVE_SIGN, GROUPING, INT_FRAC_DIGITS)
   (FRAC_DIGITS, P_CS_PRECEDES, P_SEP_BY_SPACE, N_CS_PRECEDES, N_SEP_BY_SPACE)
   (P_SIGN_POSN, N_SIGN_POSN, INT_P_CS_PRECEDES, INT_P_SEP_BY_SPACE)
   (INT_N_CS_PRECEDES, INT_N_SEP_BY_SPACE, INT_P_SIGN_POSN, INT_N_SIGN_POSN)
   [HAVE_DECL__NL_NUMERIC_GROUPING]: map to newlib C constants, when present