I have my own language, the install package is created by autotools.
Part of the package is vim syntax highlighter. How should I install it? Are there any env vars I can rely on (path to vim's install dir etc.)? Any other tips?
Vim's install process doesn't export or create any alias or environment variable. A few are usable in subshells started from Vim itself with :shell but they are gone as soon as you exit those subshells. Anyway, the directories that those elusive environment variables point to are not safe at all for, among others, the reason given by FDinoff.
If you can, tell your users to place that syntax script in $HOME/.vim/syntax/ (UNIX & Co.) or $HOME\vimfiles\syntax\ (Windows) and add the two lines below to their $HOME/.vimrc (UNIX) or $HOME\_vimrc (Windows):
filetype plugin indent on
syntax on
~/.vim. Pathogen/Vundle/NeoBundle/VAM/… are popular alternatives that provide a number of benefits, but in:h pluginVim documentation explicitly suggests using~/.vimand that is standard.