illumos#5969 breaks adjuncts build, tools
The integration of 5969 caused several problems. Most notably, it hardcoded paths into /usr without using the adjunct, causing any user of an adjunct that's not the build system on /usr, to fail. In addition, the changes to how this was constructed allow folks who attempted to use -isystem to workaround various python LFS issues to end up in trouble and be unable to work. This attempts to basically allow that to happen again as it used to, albeit rather grossly.