build: output-example: Add avutil to ELIBS in link command

output-example links against swscale, which depends on avutil.  In standard
configurations, by pure good luck, the libs before swscale pull in all of
swscale's dependencies and linking succeeds.  However, in some configurations
like --disable-asm this is not the case and linking fails.

Hardcode the dependency to avoid a more general Makefile refactoring.
avformat: Add AVPROBE_SCORE_EXTENSION define and use where appropriate
avs: Increase magic value score above file extension score

This should fix misdetection of AVS files as AviSynth scripts
when AviSynth support is enabled (Bugzilla #357).
sctp: Add a note on why sctp_recvmsg and sctp_sendmsg have ff_-prefixes
lavfi: let gcc realign the stack on public graph driving functions

The functions which actually drive the filter graph by pushing
frames through it need to ensure an aligned stack for SIMD functions.

This fixes a crash in YADIF filter when using a mingw build in a MSVC
application.

Signed-off-by: Martin Storsjö <martin@martin.st>
avutil: Add av_cold attributes to init functions missing them
swscale: Add av_cold attributes to init functions missing them
configure: Check for getenv

When targeting the "windows store application" (metro) API subset
(or the windows phone API subset), the getenv function isn't
available. If it is unavailable, just define getenv to NULL.

The check uses check_func_headers, since the function actually
might exist in the libraries, but is hidden in the headers.

The fallback is in config.h since msvc can't do -D defines with
parameters on the command line, and it's used both within the
libraries and the frontend applications (so a libavutil internal
header wouldn't be enough).

Signed-off-by: Martin Storsjö <martin@martin.st>