* Compile tests only when --enable-tests was specified on the configure command line

* Compile tests only when --enable-tests was specified on the configure command line
* Build compat layers only when --enable-compat-{howl,libdns_sd} was passed to configure
* drop avahi_strlcpy() to reduce our code/API size
* replace getifname() with if_indextoname in avahi-dnsconfd
* declare environ if needed in avahi-dnsconfd
* drop some useless definitions like  AVAHI_PUBLISH_NULL = 0

git-svn-id: file:///home/lennart/svn/public/avahi/trunk@819 941a03a8-eaeb-0310-b9a0-b1bbd8fe43fe