Welcome to the quIRC wiki!
A ‘gitless’ source tarball is one from which git calls, specifically the ‘git describe —tags’ used to generate the version number (and version.h), have been removed. This enables you to compile quIRC from source without having to clone the repository.
getaddrinfo_a() is used to provide asynchronous name lookups, preventing quIRC’s interface from locking up while waiting for a server name to resolve. If your system doesn’t have it (it’s a glibc extension), copy ‘config.mak’ from the dist/ subdirectory into the main quirc directory, and uncomment the relevant lines. Note that the old synchronous NL code may not get as thorough testing as the new asynch NL.
Another possible source of compile errors is a bug in gcc, which occurs when using a C runtime library other than glibc, for example uclibc or musl. gcc complains because it disagrees with the C runtime on what an intmax_t is, and so printfs using %jd cause warnings (which -Werror promotes to errors). There is a workaround for this, which can be enabled using config.mak, similarly to the previous case.