[bch]

I'm missing something.

Are you saying you want to be able to compile either way /usr/include/stdio.h and /usr/local/include/stdio.h, but remember what the last compilation used and know what header would be used in the next compilation, and if it's different, mark the target as stale and perform the action?

I guess you'd need to keep a log of the build and test cpp invocations for diffs.

I've never run into this scenario.

[cjhanks]

An obvious case would be a developer supporting multiple versions of a 3rd party library.

[bch]

This is where I saw the beauty of including dependencies w a project. Even on my own systems, as environments change, things break, and having a stable in-tree reference had paid off.

It's a tough situation, but I find myself leaning to @tedunangst position over the years - usually I try to adapt my machines (incl software) to my needs, but this case I need to take control/responsibility, and here be dragons. Does cmake actually solve this? Do other build systems?