Thanks for the suggestion but I want to learn how to fix the problem here rather than
install a workaround version. I may not have that option the next time I encounter a
situation like this.
I agree with you. Learning the fix is the most satisfying part. The Slackbuilds are good for that too.
Just download the slackbuild script and you can see the commands used inside it. They are the same as what you would use in a terminal when you install it to your system.
For example in the link I sent you this is how the build was done in the script.
sed -i 's/pthread/pthread -lgcrypt/' src/Makefile
# Compile the application and install it into the $PKG directory
It's just another method of doing what you managed to by searching the internet and the archlinux bulletin board. It has one advantage though. When you start getting good at slackbuilds you can then start packaging. That way you would be able to solve the xmlstarlet problem for all vl-users by making it available as a .tlz in the repository.
Anyway, I'm babbling. Glad you found your solution. Good work.