Add a more flexible script for installing a conversion utility

This changes the install-build-svg2png-tool make target to call a new
script install-tools.sh which will check whether a suitable tool is
already available and if not try and install one.

Rather than relying only on apt-get which is platform specific it will
try and use apt-get/yum/homebrew/macports as appropriate
Add ability to use yum to install-tools.sh