First I had to edit the program's makefile which was written for Solaris (however my computer runs Red Hat Linux). The top of the makefile listed what changes would be needed to get it working on a non-Solaris system and most of them pertained to changing the paths to various X-Windows libraries.
The first thing I did was reorder the list of suggestions at the top of the makefile as they were in a very illogical order. I also decided to mark all of the lines I changed with a [edited by lwb4] tag. I figured this could be a useful habit for debugging.
Then I had to find my X-Windows libraries. I think there is a bug in the Red Hat Linux filesystem search tool - its searches rarely terminate, so I had to dig around for the libraries myself. Fortunately, the paths weren't too different from the Solaris paths, so they didn't take long to find. I noticed that some of my libraries / directories had an "R6" where the versions originally in the makefile had contained an "R5," and I was a little worried that I might have a newer version of X-Windows than VPR was expecting. But if this is the case, everything seems backwards compatible, which I suppose is to be expected. Once I fixed the paths, VPR seemed to compile fine.
It was very convenient that the makefile pointed out the lines I needed to edit. I think getting VPR compiling would have taken much longer otherwise.
Although VPR appeared to have compiled, whenever I tried to run it I got an error message about no program named VPR existing or no known program being able to open a file called VPR. It turns out I had to edit the PATH variable to include "/" (the unix symbol for "current directory"). I had been assuming that you could run any program in the current directory, but it turns out this is only true if the "/" is included in your PATH, the list of places where programs you want to run can be.
Once I fixed this, VPR worked! Yay! I'd been wondering what graphics it could need to require all of the X-Windows libraries, since from everything I'd read about VPR it took a text file as an input and produced a more useful text file as an output. But when I ran VPR, it turned out to have very interesting graphics, which draw a picture of the whole FPGA and show the wires and connections it was assigning, with everything carefully color-coded. Realizing I really had no idea what all of the colored-lines VPR was presenting me with meant, I set off to read the manuals.