From ArchWiki

Those wishing to build old packages with ABS can often just grab the PKGBUILD from cvs.archlinux.org -- however, with larger and more complex packages, the required patches can be so numerous as to be a headache to download manually, especially if what you need has been moved to the attic. This is a little easier:

First, you'll need CVS (not cvsup)

pacman -S cvs

Now make a temporary directory anywhere you want, and from that directory do (you don't need to be root):

export CVSROOT=":pserver:anonymous@cvs.archlinux.org:/home/cvs-arch"

touch ~/.cvspass (necessary for some bloody reason... it needs it even if it's empty)

cvs login password is anonymous

cvs checkout -D yyyy-mm-dd arch (year-month-day -- set to whatever date of the old version you want it)

cvs logout

Now you've got a complete mirror of the cvs tree from that date. Drop whatever you want into /var/abs/local and build as usual.

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Note: There is probably a much more elegant way to go about this.