I tried to build bitcoin following the instructions in build-unix.txt. I suppose I should have read this line: "The build hasn't been updated to work with wxWidgets 2.9.1 yet."
Anyway, the makefile is not very robust in specifically including wx files directly in the INCLUDEPATHS and WXLIBS. I had a bunch of build problems which were all solved by simply using the utility that comes with wx, wx-config. This utility is automatically installed by wx's 'make install', and will provide the correct command-line arguments for including and linking correctly.
I have updated the makefile to use these tools rather than hand-coding the paths. It works for me, but I haven't tested it on multiple configurations. If it works for other people (and I figure it will be much more robust in the long run), then it should be merged.
My branch also adds .gitignore, which I find useful.