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glibc is not an extension, it is part of the base.For info:tc-10.x glibc-2.28tc-11.x glibc-2.30
A discussion of these changes is in the issue tracker; you can see the glibc patch and the new make-dynamic-linker-cache build phase. In short, the make-dynamic-linker-cache phase computes the set of direct and indirect dependencies of an ELF file using the file-needed/recursive procedure and derives from that the library search path, creates a temporary ld.so.conf file containing this search path for use by ldconfig, and finally runs ldconfig to actually build the cache.
GNU Guix is a package manager that offers transactional, reproducible, per-user package management. While Guix can be used stand-alone and provide a full GNU distribution and a kernel by itself, you can install the Guix package manager on top of Arch to make Guix available to users while using a more traditional and mature Unix-like system as a base.