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.