BUILD: install only relevant and existing documentation

doc/haproxy-{en,fr}.txt have been removed recently but they were still
referenced in the Makefile. Many other documents have also been
added. Instead of hard-coding a list of documents to install, install
all those in doc/ with some exceptions:

 - coding-style.txt is more for developers
 - gpl.txt and lgpl.txt are usually present at other places (and I would
   have to remove them in the Debian packaging, less work for me)

The documentation in the subdirectories is not installed as it is more
targeted to developers.
1 file changed