As egreg said, kpsewhich -a texmf.cnf will show you the location of the file. However, there are other ways. As I said locate texmf.cnf is maybe the simplest.
Or you can look in the PKGBUILD:
# replace upstream texmf.cnf with ours
rm -f "$pkgdir"/usr/share/texmf-dist/web2c/texmf.cnf
install -m644 "$srcdir"/texmf.cnf "$pkgdir"/etc/texmf/web2c/texmf.cnf
# since the location of texmf.cnf is hard-wired to be under /usr/share/texmf/web2c
# we make a symlink from /etc/texmf/web2c/texmf.cnf to the latter
ln -sf /etc/texmf/web2c/texmf.cnf "$pkgdir"/usr/share/texmf-dist/web2c/texmf.cnf
This shows that the file is installed in /usr/share/texmf-dist/web2c/texmf.cnf with a symlink from /etc/texmf/web2c/texmf.cnf. But don't edit /usr/share/texmf-dist/web2c/texmf.cnf - as the file itself points out, your changes will be overwritten on update if you do.
Should locate not be available, install it using
pacman -S mlocate
kpsewhich -a texmf.cnfand report the result. – egreg Apr 13 '14 at 18:10export TEXMFHOME=your_folderin your.bashrcor something similar? – Apr 13 '14 at 18:14locate texmf.cnf? – cfr Apr 13 '14 at 18:49/usr/share/texmf-dist/web2c/texmf.cnfand runfmtutil-sys --allafter changing its contents. – strpeter Mar 09 '15 at 00:08