avisynth: Change demuxer name to avoid conflicts with AVS
configure: Move newlib libc check before mingw libc check

On Cygwin systems MinGW headers can be present if the corresponding
packages have been installed. Since the MinGW libc is checked for
first, this results in newlib getting misdetected as MinGW libc.
configure: Add a comment indicating why uclibc is checked before glibc
x86: hpel: Move {avg,put}_pixels16_sse2 to hpeldsp