packaging: build recipes for arch, gentoo, nix, void, fedora

Five distro recipes in packaging/, all following the same recipe:
unpack the Gitea release archive (which extracts to fastwc/), run
autoreconf -fi (the generated configure/Makefile.in are not committed),
configure, make, make install - the binary needs only glibc.

Each recipe notes the release prerequisite: the archive URL requires a
v0.1.0 tag on the remote, and Arch/Nix/Void carry an integrity
placeholder to fill in once the tag exists. The PKGBUILD was validated
with makepkg --printsrcinfo, and the full build recipe was run
end-to-end from a Gitea-layout tarball (autoreconf, configure, make,
make install DESTDIR) producing a working /usr/bin/fastwc.

.gitignore

@@ -63,7 +63,7 @@ benchmarks/stdin/piping/FAILED-benchmark.txt
 
 # ---> fastwc build artifacts
 bin/
-fastwc
+/fastwc
 compile_commands.json
 .cache/
 
@@ -81,3 +81,6 @@ depcomp
 install-sh
 missing
 
+
+# autoconf backup from in-tree regeneration
+configure~