lavu: Provide fallbacks for gmtime_r and localtime_r

This allows writing most code as if they always are is available.

These are ok to use from other libraries even though it's not a
public header, since they only provide an inline declaration, and
doesn't add an actual dependency on lavu internals. (This can be
considered more a build system compatibility fallback than a
libavutil feature.)
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -1453,6 +1453,7 @@ SYSTEM_FUNCS="
     getservbyport
     GetSystemTimeAsFileTime
     gettimeofday
+    gmtime_r
     inet_aton
     isatty
     jack_port_get_latency_range
@@ -4062,6 +4063,7 @@ check_func  gethrtime
 check_func  getopt
 check_func  getrusage
 check_func  gettimeofday
+check_func  gmtime_r
 check_func  isatty
 check_func  localtime_r
 check_func  mach_absolute_time

libavutil/time_internal.h

+/*
+ * This file is part of Libav.
+ *
+ * Libav is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * Libav is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with Libav; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef AVUTIL_TIME_INTERNAL_H
+#define AVUTIL_TIME_INTERNAL_H
+
+#include <time.h>
+#include "config.h"
+
+#if !HAVE_GMTIME_R
+static inline struct tm *gmtime_r(const time_t* clock, struct tm *result)
+{
+    struct tm *ptr = gmtime(clock);
+    if (!ptr)
+        return NULL;
+    *result = *ptr;
+    return result;
+}
+#endif
+
+#if !HAVE_LOCALTIME_R
+static inline struct tm *localtime_r(const time_t* clock, struct tm *result)
+{
+    struct tm *ptr = localtime(clock);
+    if (!ptr)
+        return NULL;
+    *result = *ptr;
+    return result;
+}
+#endif
+
+#endif /* AVUTIL_TIME_INTERNAL_H */