I am looking to configure and compile FFmpeg with custom filter.
custom filters are available on this git repository which is compiled with older ffmpeg version
https://github.com/numberwolf/FFmpeg-PlusPlus

I am following the steps mentioned on the ffmpeg official website
https://github.com/FFmpeg/FFmpeg/blob/master/doc/writing_filters.txt

I am getting warning plusglshader filter is unknown and same for all other filters

Here are my steps:

First, create your filter source files in the libavfilter directory:

`libavfilter/vf_plusglshader.c
libavfilter/vf_pipglshader.c
libavfilter/vf_lutglshader.c
libavfilter/vf_fadeglshader.c`

Update the FFmpeg build system by adding your new filter files to libavfilter/Makefile:

I am getting following below steps

First, create your filter source files in the libavfilter directory:

`libavfilter/vf_plusglshader.c
libavfilter/vf_pipglshader.c
libavfilter/vf_lutglshader.c
libavfilter/vf_fadeglshader.c`

Update the FFmpeg build system by adding your new filter files to libavfilter/Makefile:

`OBJS-$(CONFIG_PLUSGLSHADER_FILTER)    += vf_plusglshader.o
OBJS-$(CONFIG_PIPGLSHADER_FILTER)     += vf_pipglshader.o
OBJS-$(CONFIG_LUTGLSHADER_FILTER)     += vf_lutglshader.o
OBJS-$(CONFIG_FADEGLSHADER_FILTER)    += vf_fadeglshader.o`

In libavfilter/allfilters.c, add your filter declarations:

`extern const AVFilter ff_vf_plusglshader;
extern const AVFilter ff_vf_pipglshader;
extern const AVFilter ff_vf_lutglshader;
extern const AVFilter ff_vf_fadeglshader;`

In libavfilter/filter_list.c (not allfilters.c), add your filters to the filter_list array:

`static const AVFilter * const filter_list[] = {
    // ... existing filters ...
    &ff_vf_plusglshader,
    &ff_vf_pipglshader,
    &ff_vf_lutglshader,
    &ff_vf_fadeglshader,
    NULL
};`

In libavfilter/allfilters.c, add your filter declarations:

`extern const AVFilter ff_vf_plusglshader;
extern const AVFilter ff_vf_pipglshader;
extern const AVFilter ff_vf_lutglshader;
extern const AVFilter ff_vf_fadeglshader;`

In libavfilter/filter_list.c (not allfilters.c), add your filters to the filter_list array:

`static const AVFilter * const filter_list[] = {
    // ... existing filters ...
    &ff_vf_plusglshader,
    &ff_vf_pipglshader,
    &ff_vf_lutglshader,
    &ff_vf_fadeglshader,
    NULL
};`

`