configure: Check whether MSVC requires using the C89-to-C99 converter

MSVC does support enough of C99 to work without the converter since
the 2013 version. Try to detect which version of the compiler in
the path needs to run the C99 converter or not. When the converter
is omitted, compilation time is reduced quite drastically.

Prior to this, users could still use --cc="c99conv -noconv cl"
when running MSVC 2013 to achieve the same.

This checks the version number instead of doing a normal compile
test, since this check needs to be done earlier in configure, before
the normal compile test helpers are usable.

Signed-off-by: Martin Storsjö <martin@martin.st>
Remove all Alpha architecture optimizations

Alpha has been end-of-lifed and no more test machines are available.
Deprecate obsolete XvMC hardware decoding support

XvMC has long ago been superseded by newer acceleration APIs, such as
VDPAU, and few downstreams still support it. Furthermore XvMC is not
implemented within the hwaccel framework, but requires its own specific
code in the MPEG-1/2 decoder, which is a maintenance burden.