Override file-writing builtin functions such that they refuse to modify the source tree

[ChrisKitching]

Seems like it would be a handy feature to override file(), configure_file() and friends such that they yell at you if you attempt to write to the source tree. We're commonly encountering problematic build systems that have cmake build systems that write things into the source tree (not just in the sense that the cmake object tree is inside the srcdir: I mean that, even in the case of an out-of-tree build, they actually write to the source tree - and usually have a gitignore rule to hide the madness). Yikes.

Such logic often leads to all kinds of nastiness, of course.