configure_file: canonicalize input and output path in dependencies
Represent the input file path internally in canonical form. Otherwise
multiple configure_file calls that share the same input file but specify
it relative to different directories (e.g. via ../) result in multiple
copies of the dependency on the rule to re-run CMake. This causes the
Ninja generator to emit duplicate phony build statements for these
dependencies, which generates an error with -w dupbuild=err, which
will be default in Ninja 1.9.
Also canonicalize the output path for consistency.
Add a test case.
Fixes: #18584 (closed)