--enable-skins2 should require --enable-libxml2
The skins2 feature seems to require libxml support to be compiled in however it's possible to successfully compile vlc "--enable-skins2 --disable-libxml2". This combination leads to skins not working and VLC failing to even start up with skins2 configured:
$ vlc
VLC media player 2.2.1 Terry Pratchett (Weatherwax) (revision 2.2.1-0-ga425c42)
[0000000000a3a248] core xml reader error: XML reader not found
[0000000000a447e8] core interface error: XML provider not found
[0000000000a447e8] skins2 interface error: cannot initialize xml
[0000000000a447e8] skins2 interface error: error while parsing /tmp/vltu2OJjZ/winamp5/theme.xml
[0000000000a447e8] core interface error: XML provider not found
[0000000000a447e8] skins2 interface error: cannot initialize xml
[0000000000a447e8] skins2 interface error: no skins found : exiting
I suggest making skins2 support dependent on libxml. This was detected downstream in Gentoo (https://bugs.gentoo.org/show_bug.cgi?id=563398) where we'll most likely make the "skins" USE flag dependent on the "xml" USE flag.