configure: add strip flags checks

This will check if -wN '..@*' is available and fall back on -x if not;
when none are available, do not run strip at all to prevent removing
functions that might be actually needed.
dsputil/pngdsp: fix signed/unsigned type in end comparison

Fixes out of array accesses and integer overflows.
x86: dsputil: Suppress deprecation warnings for XvMC bits

These parts are scheduled for removal on the next version bump.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>