libavformat/hlsenc: default segment name and use_localtime

in hlcenc.c, in the hls_write_header() function the default format
string for strftime() function contains %s specifier when use_localtime
is true. This %s specifier will insert the seconds since EPOCH. But %s
is not available on all system/environment. This patch check %s
availabilty at runtine and alter the default format string if necessary.

Signed-off-by: Bela Bodecs <bodecsb@vivanet.hu>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>
