alsa-audio-dec: explicitly cast the delay to a signed int64

Otherwise the expression will be evaluated as unsigned, which will break
when the result should be negative.
CC:libav-stable@libav.org
avconv: initialize hwaccel_pix_fmt

Fixes an assertion failure in vdpau_get_buffer() when -hwaccel auto is
used, but hwaccel init fails.
configure: Add detected C library and host compiler to informative output
configure: Detect msvcrt libc with a CPP check instead of a link check

Simplifies host/target libc detection splitting.