Move documentation files to /usr/share
As requested by Matt Gushee on the mailing list, documentation files (/usr/lib/pure/docs) should be moved to /usr/share. This entails the following changes:
Fix up make install and uninstall targets in pure/Makefile.
Fix up the file search logic in the help command (lexer.ll), as well as the Emacs and TeXmacs modes, so that the documentation search path is derived from the PURELIB prefix in an automatic fashion.
Add a datadir variable to pure.pc which points to the /usr/share/pure directory, so that addon packages can find the new install location for documentation and other platform-independent files.