[CMake] Behaviour of CACHE PATH variables

[Brad King]

On 03/25/2015 10:15 AM, Roger Leigh wrote:
> Question: Why is TESTDIR2 transformed from "t2" to
> "/home/rleigh/pathtest/test/t2" (which is
> "$PROJECT_BINARY_DIR/$TESTDIR2)?

It looks like the behavior has been this way since 2003:

 ENH: When fixing cache value with uninitialized type, collapse
 full paths for PATH and FILEPATH. Closes Bug #82 - Specifying
 relative path when entering path can break things
 http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=a6567329

but was added without documentation back in our wild west days.

> Copying the GNUInstallDirs behaviour of wrapping in an "if (NOT DEFINED foo)" test
> mitigates the problem.
> 
> For my usage, I'm defining project-specific install dirs derived from the
> toplevel GNUInstallDirs dirs.  I'd quite like to be able to specify a relative
> (or absolute) path, and then later prepend the install prefix / appropriate
> GNUInstallDirs toplevel variable.  Having the current working directory added
> by default screws this up quite badly!

>From the commit message above I think the intended behavior is
exactly to transform relative paths.  The behavior has been this
way for so long I don't think we can change it now.  We can update
the documentation to describe the behavior, but I think using the
approach in GNUInstallDirs is the solution for you.

-Brad