It was rather confusing that XPM, Xinerama and Xext seemed absolutely
required even though only skins2 depends on them. Also the check for
libtar is now skipped if skins2 are disabled.

This works when running straight OpenMAX on a rooted Nexus One
with chmodded /dev nodes.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

 There is an incorrect interaction with 'pause on minimized' feature
when showing back the video (depending on qt-system-tray).

The interfaces that want to implement it can now add a callback on
libvlc::intf-boss.

 - use native MingW back-end on Windows, instead of kludges,
 - remove "base" pointer parameter,
 - inline.

It closes #5053.

It closes #4695.

It is send on vout count changes (close #5277).

 - only check for libv4l2 if V4L2 is supported,
 - only check for --enable-pvr if V4L2 is supported,
 - remove mostly useless --enable-libv4l2 and --enable-v4l2.

Save CFLAGS and CXXFLAGS after the compiler is detected, rather than
before. This is when autoconf sets -g and -O2 automatically by default.

Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

Untested