build: clean up remnants from internal FFmpeg build

build: clean up remnants from internal FFmpeg build

Clean up some obsolete configure tests and other things left over from
the time years ago when MPlayer used hacks to build FFmpeg inside the
same source tree.

Remove the configure switches --as, --ar, --ranlib. --ar and --ranlib
were useless since nothing is built as a library. --as wasn't directly
used; removing it means removing some sanity checks though (the
default assembler selected was tested to see that it could compile SSE
opcodes, which may be used in inline asm).

It's unlikely that anyone would try to make the filters depending on
Libavcodec internals build any more. Remove the undocumented configure
option --fmpeg-source-dir and remove the mcdeint and spp filters from
the source tree (they were already not built).

Remove some configure tests which were added for the sake of FFmpeg
build and whose results are not used by mplayer2 now.

Removing the memalign checks actually required removing two remaining
uses in the code. Also remove the malloc.h check and includes in code
(I guess the reason to add malloc.h includes at some point was
memalign use, though most of the files in question did not actually
use memalign now, if ever).