configure.ac: backport [24902] use build_os instead of SYS for detecting cygwin for hostname check because when we deal with mingw32 compiler (mno-cygwin mode) then SYS value isn't cygwin but (forced to) mingw32

Backport [23166] [23170] [23182] [23183] [23483] [23526] [23527] [23596]: Several live555 demuxer fixes.

* modules/demux/ogg.c: backport [24258] fix for demuxing Ogg with skeleton track. As noted in #1412 all content in Ogg now require a skeleton track..

* configure.ac, modules/gui/wxwidgets/interface.cpp: backport (where it applied to wx) [23985] COPYRIGHT_YEARS, [24271] hostname -s and [24272] HAVE_LIBPNG

Patch by Christopher Key. Attached is a patch that fixes a long standing problem whereby video encoded with vlc using x264 cannot be made playable on an iPhone. The issue was traced to the H264 profile value being incorrectly set, which this patch addresses.  Looking at the code, the trunk appears to have had a complete rewrite and should be unaffected, but this may be useful for a 0.8.6e release.