MSVC: Add abstraction for runtime library selection

Replace our hard-coded defaults for `/MD` and `/MDd` with a first-class
abstraction to select the runtime library from an enumeration of logical
names.  We've long hesitated to do this because the idea of "runtime
library selection" touches on related concepts on several platforms.
Avoid that scope creep by simply defining an abstraction that applies
only when targeting the MSVC ABI on Windows.

Removing the old default flags requires a policy because existing
projects may rely on string processing to edit them and choose a runtime
library under the old behavior.  Add policy CMP0091 to provide
compatibility.

Fixes: #19108

1 files changed, 27 insertions, 0 deletions

diff --git a/Help/variable/CMAKE_MSVC_RUNTIME_LIBRARY.rst b/Help/variable/CMAKE_MSVC_RUNTIME_LIBRARY.rst
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index 0000000..6ed68c9
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+++ b/Help/variable/CMAKE_MSVC_RUNTIME_LIBRARY.rst
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+CMAKE_MSVC_RUNTIME_LIBRARY
+--------------------------
+
+Select the MSVC runtime library for use by compilers targeting the MSVC ABI.
+This variable is used to initialize the :prop_tgt:`MSVC_RUNTIME_LIBRARY`
+property on all targets as they are created.  It is also propagated by
+calls to the :command:`try_compile` command into the test project.
+
+The allowed values are:
+
+.. include:: ../prop_tgt/MSVC_RUNTIME_LIBRARY-VALUES.txt
+
+Use :manual:`generator expressions <cmake-generator-expressions(7)>` to
+support per-configuration specification.  For example, the code:
+
+.. code-block:: cmake
+
+  set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
+
+selects for all following targets a multi-threaded statically-linked runtime
+library with or without debug information depending on the configuration.
+
+.. note::
+
+  This variable has effect only when policy :policy:`CMP0091` is set to ``NEW``
+  prior to the first :command:`project` or :command:`enable_language` command
+  that enables a language using a compiler targeting the MSVC ABI.