add a osx benchmark

'time' is very non-linux on os x, make a separate benchmark

benchmark/benchmarkn-osx.sh

@@ -0,0 +1,45 @@
+#!/bin/bash
+
+uname -a
+gcc --version
+vips --version
+
+# how large an image do you want to process? 
+# sample2.v is 290x442 pixels ... replicate this many times horizontally and 
+# vertically to get a highres image for the benchmark
+tile=13
+
+# how complex an operation do you want to run?
+# this sets the number of copies of the benchmark we chain together:
+# higher values run more slowly and are more likely to be CPU-bound
+chain=1
+
+echo building test image ...
+echo "tile=$tile"
+vips im_replicate sample2.v temp.v $tile $tile
+if [ $? != 0 ]; then
+  echo "build of test image failed -- out of disc space?"
+  exit 1
+fi
+echo -n "test image is" `header -f width temp.v` 
+echo " by" `header -f height temp.v` "pixels"
+max_cpus=`vips im_concurrency_get`
+
+echo "max cpus = $max_cpus"
+echo "starting benchmark ..."
+echo /usr/bin/time vips \
+  --vips-concurrency=xx \
+  im_benchmarkn temp.v temp2.v $chain
+
+for((cpus = 1; cpus <= max_cpus; cpus++)); do
+  echo cpus = $cpus 
+  /usr/bin/time vips \
+	  --vips-concurrency=$cpus \
+	  im_benchmarkn temp.v temp2.v $chain 2>&1
+  /usr/bin/time vips \
+	  --vips-concurrency=$cpus \
+	  im_benchmarkn temp.v temp2.v $chain 2>&1
+  /usr/bin/time vips \
+	  --vips-concurrency=$cpus \
+	  im_benchmarkn temp.v temp2.v $chain 2>&1
+done