Say you are writing a program to edit images that can potentially read lots of different formats. However most people don't need support for .sgi or .ecw files and won't have the libraries to read those formats installed. Your program is still perfectly usable without those libraries and so you should still be able to compile and run it, just without support for those formats.
Or even worse, it is used as a dependency in a script, or it is a library linked by another program, but the author can never trust that any given installation has the feature he actually needs.
It is a mess. Just don't do it.
By the way another thing I've come to hate is configure or make scripts which download stuff. This is really annoying if you want to build self-contained packages. Or if the downloaded material ends up being not available, or if you're not always connected to the internet.
If it existed, it would be a nightmare, because features are recursive, and you would end up with fragile monstrosities like
foo[+bar[+baz[+qux],+quux]]
¹ https://devmanual.gentoo.org/general-concepts/dependencies/i...
² https://setuptools.readthedocs.io/en/latest/userguide/depend...
* https://guide.macports.org/chunked/reference.variants.html
* https://guide.macports.org/chunked/using.variants.html
The default / most common variants have pre-built binaries that are downloaded via a port install, but if you specify something more 'obscure', things are built from source.
FreeBSD's Ports also allow for options:
* https://docs.freebsd.org/en/books/porters-handbook/#makefile...
Build-time and run-time dependencies are both handled.
That's the "glass half empty" pov.
The "half glass full" pov is that anyone can still use their favourite piece of software even if an obscure and/or useless library is not available to them.
Sure, for hobbyists it is fine but professional use.. I disagree.