AC_STRUCT_TIMEZONE tzname on mingw

[Kevin Ryde]

        * lib/autoconf/types.m4 (AC_STRUCT_TIMEZONE): Test for tzname with
        AC_CHECK_DECL before giving own declaration in test program.  Fixes
        detection of tzname on mingw.  On mingw an explicit "extern char
        *tzname[];" overrides import directives in <time.h>, making the test
        fail to link and (wrongly) imply tzname doesn't exist.

This is a bit of a case of autoconf not following its own advice about
giving explicit "extern" declarations. :-)

I've only actually tried this with recent debian packaged mingw cross
tools, so maybe someone with an actual mingw can confirm.

The sort of thing the test program uses,

        #include <time.h>
        extern char *tzname[];
        int
        main (void)
        {
          atoi (*tzname);
          return 0;
        }

gets errors

        Info: resolving _tzname by linking to __imp__tzname (auto-import)
        fu000001.o(.idata$3+0xc): undefined reference to `libmoldname_a_iname'
        nmth000000.o(.idata$4+0x0): undefined reference to `_nm__tzname'

whereas the following links successfully

        #include <time.h>
        int
        main (void)
        {
          atoi (*tzname);
          return 0;
        }

(both just as "i586-mingw32msvc-gcc foo.c").


--- types.m4.~1.26.~    2004-03-25 08:44:51.000000000 +1000
+++ types.m4    2004-03-25 08:44:59.000000000 +1000
@@ -614,10 +614,14 @@
            [Define to 1 if your `struct tm' has `tm_zone'. Deprecated, use
             `HAVE_STRUCT_TM_TM_ZONE' instead.])
 else
+  # On SGI, apparently tzname is a #define, but that's ok, AC_CHECK_DECL will
+  # consider it declared and we won't give our own extern.
+  AC_CHECK_DECL(tzname, ac_tzname_decl_test=1, ac_tzname_decl_test=0,
+                [#include <time.h>])
   AC_CACHE_CHECK(for tzname, ac_cv_var_tzname,
 [AC_LINK_IFELSE([AC_LANG_PROGRAM(
 [[#include <time.h>
-#ifndef tzname /* For SGI.  */
+#if ! $ac_tzname_decl_test
 extern char *tzname[]; /* RS6000 and others reject char **tzname.  */
 #endif
 ]],