multimedia/vlc: fix build with CHROMECAST ON and devel/protobuf 24 and fix...

multimedia/vlc: fix build with CHROMECAST ON and devel/protobuf 24 and fix plist error if QT5 is ON and xext,xinerama,xpm installed

1. Non default option CHROMECAST require devel/protobuf.
After this commit
https://cgit.freebsd.org/ports/commit/?id=69e2e87fa56b54e267429b326f7f6188a7baaa71
consumers require USE_CXXSTD=c++17.
2. There is default ON configure option "skins2" in vlc. It require QT5 and
xext,xinerama,xpm - if they installed then it build bin/svlc and it is in
stage after build. Added option SKINS default OFF to handle bin/svlc.

Approved by:		arrowd (mentor)
Differential Revision:	https://reviews.freebsd.org/D43128