Add and use a couple helper functions for issuing policy warnings. This
improves consistency and allows some simplification of many call sites.
(One or two instances in particular are greatly simplified.)
Add new diagnostic categories CMD_POLICY and CMD_EXPERIMENTAL, both as
children of CMD_AUTHOR. Use the former when issuing policy warnings. Use
the latter when issuing experimental feature warnings.
Tweak `cmMessenger` to also show the "warning is for project developers"
notice for all diagnostics that are descendants of the `CMD_AUTHOR`
category. When doing so, also show both command-line options which can
suppress the message.
Update expected test output to account for the new format of diagnostic
messages. These changes were applied entirely by `sed` using the
following commands:
- `/Use -Wno-dev to suppress/s/-Wno-dev/-Wno-author/g`
- `s/Warning [\\][(]dev[\\][)]/Warning \\(author\\)/g`
- `s/CMake Deprecation Warning/CMake Warning \\(deprecated\\)/g`
This is not a complete fix, but covers a significant fraction of the
needed changes, and this commit is strictly automated changes.
The OLD behaviors of all policies are deprecated, but only by
documentation. Add an explicit deprecation diagnostic for policies
introduced in CMake 3.20 and below to encourage projects to port
away from setting policies to OLD.
For policy-specific tests, use the version before the policy was
introduced. Otherwise, use 3.5 where possible.
Also, remove `cmake_minimum_required()` and `project()` calls from
individual cases where they are handled by `CMakeLists.txt`.
This allows defining compat versions of some C/C++ features with the name of the
keyword itself, so all code can look as if it was written for the new language
standard.
Extend the write_compiler_detection_header interface to allow
specifying a location for supplementary files, and getting the
list of resulting files as a variable.
Provide a function to write a portable header to detect compiler
features. Generate a preprocessor #error for unknown compilers
and compiler versions whose features are not yet recorded. This
error condition might be relaxed in the future, but for now it
is useful for verification of expectations.
