is that a safe file to edit?
There's no need to edit any files in this case. What the error is saying, is that a package description file (jack.pc) wasn't found in any of the expected locations.
This is caused by one of three reasons (usually). In order of things to check:
1) The jack library/program is not installed. Make sure you have the software ardour is looking for.
2) This is your first time to compile from source (or relatively close), using a distribution with package management. Those distributions do not install all the support files needed to compile other programs. The reason for this is, they expect you to use the package management system. That's the whole reason it exists: to hide the details of compiling. So, check to see if there is a package for ardour available for Slackware. If there isn't, then you will probably have to install a "jack-devel" package from your install CD's or from wherever you got jack to begin with. The "-devel" packages contain the support files that other programs need to hook into jack, or other libraries/tools. Keep in mind, that you will have to do that for any other packages the configure script claims you're missing.
3) Your system has jack installed in a non-standard location. You'll have to inform the system (or the configure script directly) where jack's .pc file is located. If this is the case, then you knew what you were doing when you put it there, which means you would probably know how to fix it
Of the three, #2 is most likely.