syslog.lua is not executable

Using _DATA instead of _SCRIPTS avoid to install it as an executable.
Avoid versioning lsyslog.so
Correct typo in lsyslog.la removal
build: improve autotools installation

Install example.lua in pkgdatadir and properly remove lsyslog.so.
rockspec: use manual file installation

Installing via "install_command = 'make install'" does not keep track of
the installed stuff, so "luarocks remove" does not do a clean uninstall.

Leverage build.install instead, so luarocks knows which files are
installed and will be able to uninstall them properly.
build: bump to 2.0.6