[pan2] Fix the strftime test for mingw64

[Petr Kovář <pmkovar src gnome org>]

commit 1c4598aedd71bc5f400fd4706960d9c81aae0af5
Author: Steve Davies <davies147 gmail com>
Date:   Wed Apr 26 00:39:57 2017 +0200

    Fix the strftime test for mingw64
    
    mingw64's strftime returns an empty string when the %l and %h
    tokens are not understood.

 configure.ac |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index ccc1ea9..2051e97 100644
--- a/configure.ac
+++ b/configure.ac
@@ -260,13 +260,14 @@ dnl Check to see if strftime supports the use of %l and %k
 AC_MSG_CHECKING(for %l and %k support in strftime)
 AC_RUN_IFELSE([AC_LANG_SOURCE([[
 #include <string.h>
+#include <stdlib.h>
 #include <time.h>
 int main(int argc, char **argv) {
   char buf[10];
   time_t rawtime = time(0);
   struct tm *timeinfo = localtime (&rawtime);
   strftime(buf, 10, "%l %k", timeinfo);
-  exit (strstr(buf, "l") || strstr(buf, "k") || !strcmp(buf," "));
+  exit (strstr(buf, "l") || strstr(buf, "k") || !strcmp(buf," ") || !strlen(buf));
 }]])],[
 AC_DEFINE(HAVE_LKSTRFTIME, 1, [strftime supports use of l and k])
 ac_cv_lkstrftime=yes