Update configure.ac

add gmtime check

configure.ac

@@ -766,6 +766,22 @@ AC_LINK_IFELSE([AC_LANG_SOURCE([
 )
 LDFLAGS="$TEMP_LDFLAGS"
 
+dnl check for gmtime_r(), fallback to gmtime_s() if that is unavailable
+dnl fail if neither are available.
+AC_MSG_CHECKING(for gmtime_r)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <ctime>]],
+  [[ gmtime_r((const time_t *) nullptr, (struct tm *) nullptr); ]])],
+  [ AC_MSG_RESULT(yes); AC_DEFINE(HAVE_GMTIME_R, 1, [Define this symbol if gmtime_r is available]) ],
+  [ AC_MSG_RESULT(no);
+    AC_MSG_CHECKING(for gmtime_s);
+    AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <ctime>]],
+       [[ gmtime_s((struct tm *) nullptr, (const time_t *) nullptr); ]])],
+       [ AC_MSG_RESULT(yes)],
+       [ AC_MSG_RESULT(no); AC_MSG_ERROR(Both gmtime_r and gmtime_s are unavailable) ]
+    )
+  ]
+)
+
 # Check for different ways of gathering OS randomness
 AC_MSG_CHECKING(for Linux getrandom syscall)
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <unistd.h>