Make the c and ac scripts work with sh and without bin in PATH

Make the c and ac scripts work with sh and without bin in PATH

Make the bin/c and bin/ac scripts work with sh. The scripts are run with
/bin/sh shebang, which fails atleast on ubuntu 12.04 /bin/sh. The sh in
Ubuntu 12.04 is dash.

The fixes are according to the suggestions in http://mywiki.wooledge.org/Bashism

Also run "compare" script with explicit ./bin/ path to support people
who do not have skia/bin in PATH.

Review URL: https://codereview.chromium.org/1139033005