Apple platforms macOS, iOS, tvOS, and watchOS can all share the same
compiler information. Rename `Darwin-*` modules to `Apple-*` and load
them all through `CMAKE_EFFECTIVE_SYSTEM_NAME`. This saves duplication
of 4 * 21 compiler information modules.
Issue: #17870
Create a `CMAKE_EFFECTIVE_SYSTEM_NAME` variable to use for looking
up compiler information modules instead of using `CMAKE_SYSTEM_NAME`
directly. This will allow multiple platforms to share the same set
of compiler information modules without spelling out all of them.
Issue: #17870
Per-source copyright/license notice headers that spell out copyright holder
names and years are hard to maintain and often out-of-date or plain wrong.
Precise contributor information is already maintained automatically by the
version control tool. Ultimately it is the receiver of a file who is
responsible for determining its licensing status, and per-source notices are
merely a convenience. Therefore it is simpler and more accurate for
each source to have a generic notice of the license name and references to
more detailed information on copyright holders and full license terms.
Our `Copyright.txt` file now contains a list of Contributors whose names
appeared source-level copyright notices. It also references version control
history for more precise information. Therefore we no longer need to spell
out the list of Contributors in each source file notice.
Replace CMake per-source copyright/license notice headers with a short
description of the license and links to `Copyright.txt` and online information
available from "https://cmake.org/licensing". The online URL also handles
cases of modules being copied out of our source into other projects, so we
can drop our notices about replacing links with full license text.
Run the `Utilities/Scripts/filter-notices.bash` script to perform the majority
of the replacements mechanically. Manually fix up shebang lines and trailing
newlines in a few files. Manually update the notices in a few files that the
script does not handle.
Create a Modules/CMakeSystemSpecificInitialize.cmake module loaded after
CMakeSystem.cmake but before per-language initialization. Use it to
load an optional Platform/<os>-Initialize.cmake module. This will be
useful to do per-platform initialization that does not depend on the
language and use the results when enabling specific languages.
