Avoid using the altzone variable in AIX

In AIX, altzone exists in the standard library but is not declared
in time.h.  By 524513b, have_var
and try_var in mkmf recognizes a variable that exists in a library
even when it is not declared.  As a result, in AIX, HAVE_ALTZONE
is defined, but compile fails due to the lack of the declaration.

ext/date/date_core.c

@@ -7797,7 +7797,7 @@ datetime_s_now(int argc, VALUE *argv, VALUE klass)
 #ifdef HAVE_STRUCT_TM_TM_GMTOFF
     of = tm.tm_gmtoff;
 #elif defined(HAVE_TIMEZONE)
-#ifdef HAVE_ALTZONE
+#if defined(HAVE_ALTZONE) && !defined(_AIX)
     of = (long)-((tm.tm_isdst > 0) ? altzone : timezone);
 #else
     of = (long)-timezone;