Avoid implicit ints in the guile_cv_localtime_cache check.  It cause
the check to always fail with strict(er) C99 compilers that do not
support implicit int.

Submitted upstream:

  <https://lists.gnu.org/archive/html/bug-guile/2022-12/msg00017.html>
  <https://debbugs.gnu.org/cgi/bugreport.cgi?bug=60022>

diff --git a/configure b/configure
index e2816d1ead4b34ba..f541916fc08f88f6 100755
--- a/configure
+++ b/configure
@@ -52466,7 +52466,7 @@ else
 # include <stdlib.h>
 #endif
 extern char **environ;
-unset_TZ ()
+int unset_TZ ()
 {
   char **from, **to;
   for (to = from = environ; (*to = *from); from++)
@@ -52475,7 +52475,7 @@ unset_TZ ()
 }
 char TZ_GMT0[] = "TZ=GMT0";
 char TZ_PST8[] = "TZ=PST8";
-main()
+int main()
 {
   time_t now = time ((time_t *) 0);
   int hour_GMT0, hour_unset;
diff --git a/configure.ac b/configure.ac
index 0a7b29045b9ac5ec..50d024d7ec057073 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1086,7 +1086,7 @@ AC_RUN_IFELSE([AC_LANG_SOURCE([[#include <time.h>
 # include <stdlib.h>
 #endif
 extern char **environ;
-unset_TZ ()
+int unset_TZ ()
 {
   char **from, **to;
   for (to = from = environ; (*to = *from); from++)
@@ -1095,7 +1095,7 @@ unset_TZ ()
 }
 char TZ_GMT0[] = "TZ=GMT0";
 char TZ_PST8[] = "TZ=PST8";
-main()
+int main()
 {
   time_t now = time ((time_t *) 0);
   int hour_GMT0, hour_unset;