kea: fix compilation with libcxx

Boost headers try to include experimental/string_view when std is less
than c++17. This does not work ith libcxx where this header is not
present.

Refreshed patches.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

net/kea/patches/004-replace-rev-with-awk.patch

@@ -6,6 +6,6 @@
      local conf_name
 -    conf_name=$(basename -- "${kea_config_file}" | rev | cut -f2- -d'.' | rev)
 +    conf_name=$(basename -- "${kea_config_file}" | awk '{for(i=length($0); i>0;i--) printf (substr($0,i,1));}' | cut -f2- -d'.' | awk '{for(i=length($0); i>0;i--) printf (substr($0,i,1));}')
-
+ 
      # Default the directory to --localstatedir / run
      local pid_file_dir