Add feature flag to use system libdeflate

[shssoichiro]

This need was raised in https://bugs.gentoo.org/944285. Can be enabled with --features system-libdeflate to use the libdeflate located on the user's system instead of building our own.

[AlexTMjugador]

Looks good to me, allowing users to choose which libraries Oxipng links to is a welcome concept. Thanks!

As a side note, the underlying Gentoo packaging issue shouldn't occur with Oxipng 9.1.3. This version updates libdeflater to 1.22, and according to the related PR, the problem stemmed from the older libdeflate version.

[AlexTMjugador]

cc @lo48576 in case the information above about internal libdeflater updates on Oxipng 9.1.3 is useful in some way for Gentoo packaging.

[lo48576]

Thank you for introducing the flag!

I'm aware of updated libdeflater dependency in the latest version. I'll use a patch for v9.1.3 and system-libdeflate flag for the later versions (because it is (maybe) more preferred way in Gentoo Linux).

The Gentoo repo has libdeflate package (app-arch/libdeflate) but currently the last version added is 1.21, and oxipng 9.1.3 depends on libdeflater 1.22.0, so I'll wait for =app-arch/deflate-1.22 to be added to the Gentoo.