Imagemagick: check for valid installation

[brother7]

When installing Coppermine, the installation script checks the Imagemagick path (if entered) and gives an error if it isn't found.

Such a check doesn't occur if changes are made in the Admin Config screen, after installation.  It would be helpful to perform a check here.

In my case, my host did not have Imagemagick installed, so I chose GD2 during installation.  They've now installed Imagemagick.  I'd like for the script to tell me if Imagemagick exists, instead of me having to test it live.

[Joachim Müller]

an advanced check for image libraries both in install and config is being scheduled.