benchmarks: OSX compat: replace /usr/bin/time with bash builtin

On OSX, /usr/bin/time does not support "-f".

Reported in https://github.com/rfjakob/gocryptfs/issues/15#issuecomment-279130217 .

benchmark-reverse.bash

@@ -35,9 +35,13 @@ trap "rm -f $PLAIN/.gocryptfs.reverse.conf ; fuse-unmount -z $MNT ; rmdir $MNT"
 # Mount
 gocryptfs -q -reverse -extpass="echo test" $PLAIN $MNT
 
-# Print elapsed wall time only
+# Execute command, discard all stdout output, print elapsed time
+# (to stderr, unfortunately).
 function etime {
-	LC_ALL=C /usr/bin/time -f %e 2>&1 $@ > /dev/null
+	# Make the bash builtin "time" print out only the elapse wall clock
+	# seconds
+	TIMEFORMAT=%R
+	time "$@" > /dev/null
 }
 
 echo -n "LS: "

tests/canonical-benchmarks.bash

@@ -20,8 +20,13 @@ fi
 # cd to TESTDIR
 cd "$1"
 
+# Execute command, discard all stdout output, print elapsed time
+# (to stderr, unfortunately).
 function etime {
-	LC_ALL=C /usr/bin/time -f %e 2>&1 $@ > /dev/null
+	# Make the bash builtin "time" print out only the elapse wall clock
+	# seconds
+	TIMEFORMAT=%R
+	time "$@" > /dev/null
 }
 
 echo -n "WRITE: "