Use $0/../share/… as a default search path for --data-dir [feature request]

[sjackman]

When Pandoc is moved after installation, it would be really helpful for Pandoc to search $(realpath $0)/../share/… for its data directory (where $0 is argv[0]). My motivating use case is when installing a binary Pandoc package in my home directory using the Homebrew for Linux package manager. The workaround of creating a ~/.pandoc symlink is pretty straight forward, but it would be fantastic if it worked out of the box. Compiling Pandoc from source also works, but it's much faster to use a binary package.

$ brew install --force-bottle pandoc
$ pandoc -o foo.html foo.md
Could not find data file /home/linuxbrew/.linuxbrew/Cellar/pandoc/2.9.1.1/share/x86_64-linux-ghc-8.8.1/pandoc-2.9.1.1/data/abbreviations
$ ln -s ~/.linuxbrew/Cellar/pandoc/2.9.1.1/share/x86_64-linux-ghc-8.8.1/pandoc-2.9.1.1/data ~/.pandoc
$ pandoc -o foo.html foo.md
# Success!

[jgm]

This is a problem with the way homebrew is building pandoc. It should use the embed_data_files flag so that the data files are baked in. (Background: in the old days cabal had nice support for installing data files to the right place, and pandoc could find them, but this is broken in stack. See commercialhaskell/stack#848. Embedding the data files fixes the problem.)
I would suggest reporting this to homebrew.

I don't want to implement your suggestion; it strikes me as potential a security risk.

[sjackman]

That's very helpful! Thank you, John.