That’s the default value defined in the system Config.prc, that’s a special token that means to search relative to wherever libdtool.so was loaded.
It also means that it didn’t read your personal Config.prc file. Probably that directory wasn’t on the search path for Config.prc files.
Try:
print ConfigPageManager.getGlobalPtr().getSearchPath()to show the search path for Config.prc files. This search path is partly determined by parameters passed to the Panda build script, and partly determined by environment variables.
David