merge: always build features which have no build deps

 * configure: drop --enable-compressed
 * configure: drop --enable-imap
 * configure: need_md5 is now always built-in
 * configure: drop need_imap
 * configure: drop need_socket
 * configure: drop --enable-pop
 * configure: drop --enable-nntp
 * drop: --enable-smtp
 * Makefile.am: reflow lists of files
 * remove preprocessor else clauses