Remove IsWritingApp function, since it wasn't being used.

Silences a gcc warning.
Insert LoadCPlugin into FFMS2.avsi to facilitate autoloading.
Expose GCC optlevel selection to configure.

Default optlevel is set at zero, to avoid MinGW-related issues
with functions in matroskaparser.c.

Also forces the use of optlevel zero for --enable-debug.
Clean up build output

Largely based on the method used for libutvideo, with fixes for .depend
Add install-avs option

Instead of the typical installation of 'make install', 'make install-avs'
only deals with the pieces relevant for making a distro package for the
AviSynth plugin:

ffms2.dll
ffmsindex.exe
FFMS2.avsi
COPYING.GPLv3
/doc
Remove postproc support from C-plugin to match trunk.
VapourSynth plugin compilation support for the non-autotools buildsystem.
Change PixelFormat to AVPixelFormat in the C plugin
Set VS plugin to export symbols under MINGW
Properly initialize VapourSynthPluginInit under MINGW
Really fix the detection of MINGW for VapourSynthPluginInit without
breaking non-Windows support.
Move to av_opt_set_int. Resolves the deprecation warnings for
 src/avisynth_c/avs_utils.c.
Add AvxSynth plugin support to the non-autotools build system.

This makes it possible to update avxffms2 without building all of
AvxSynth, as well as dual-purpose AvxSynth/VapourSynth plugins,
like the existing AviSynth/VapourSynth capability.
Cosmetic changes to configure's options.

--enable-avisynth and --enable-avxsynth are added. The short named
options (--enable-vs has been added to these) won't be removed,
but not mentioned in the help.

The configure report now also explicitly says 'avisynth' and
'avxsynth'.