Support for dynamic-module build

[jengelh]

./configure --add-dynamic-module=/zstd
[   20s] configuring additional dynamic modules
[   20s] adding module in /zstd
[   20s]     ./configure: error: ngx_http_zstd_filter_module can only be linked as an addon.
[   20s] checking for ZStandard static library ... not found
[   20s] checking for ZStandard dynamic library ... found

Would it be possible to support dynamic mode? ngx_brotli for example can do dynamic, so it seems that filters don't have a restriction to be hard-builtin.

[tokers]

It's feasible, I'll try to support it as soon as possible, if you're interested in it, welcome to submit a PR :)

[eilandert]

this should now be possible