Make use of zlib optional

Fio uses it to compress io logs between the client and server.
But let them handshake on this requirement when they connect
and probe, and fallback to just sending the logs in plain
format if both of them do not support zlib compression.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/configure b/configure
index 87d659c..7be8789 100755
--- a/configure
+++ b/configure
@@ -415,9 +415,6 @@
 if compile_prog "" "-lz" "zlib" ; then
   zlib=yes
   LIBS="-lz $LIBS"
-else
-  feature_not_found "zlib" "zlib development files"
-  zlib=no
 fi
 echo "zlib                          $zlib"
 
@@ -1083,6 +1080,9 @@
 else
   output_sym "CONFIG_LITTLE_ENDIAN"
 fi
+if test "$zlib" = "yes" ; then
+  output_sym "CONFIG_ZLIB"
+fi
 if test "$libaio" = "yes" ; then
   output_sym "CONFIG_LIBAIO"
 fi