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, 47 insertions, 0 deletions

diff --git a/Help/policy/CMP0091.rst b/Help/policy/CMP0091.rst
new file mode 100644
index 0000000..5b7c4e3
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+++ b/Help/policy/CMP0091.rst
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+CMP0091
+-------
+
+MSVC runtime library flags are selected by an abstraction.
+
+Compilers targeting the MSVC ABI have flags to select the MSVC runtime library.
+Runtime library selection typically varies with build configuration because
+there is a separate runtime library for Debug builds.
+
+In CMake 3.14 and below, MSVC runtime library selection flags are added to
+the default :variable:`CMAKE_<LANG>_FLAGS_<CONFIG>` cache entries by CMake
+automatically.  This allows users to edit their cache entries to adjust the
+flags.  However, the presence of such default flags is problematic for
+projects that want to choose a different runtime library programmatically.
+In particular, it requires string editing of the
+:variable:`CMAKE_<LANG>_FLAGS_<CONFIG>` variables with knowledge of the
+CMake builtin defaults so they can be replaced.
+
+CMake 3.15 and above prefer to leave the MSVC runtime library selection flags
+out of the default :variable:`CMAKE_<LANG>_FLAGS_<CONFIG>` values and instead
+offer a first-class abstraction.  The :variable:`CMAKE_MSVC_RUNTIME_LIBRARY`
+variable and :prop_tgt:`MSVC_RUNTIME_LIBRARY` target property may be set to
+select the MSVC runtime library.
+
+This policy provides compatibility with projects that have not been updated
+to be aware of the abstraction.  The policy setting takes effect as of the
+first :command:`project` or :command:`enable_language` command that enables
+a language whose compiler targets the MSVC ABI.
+
+.. note::
+
+  Once the policy has taken effect at the top of a project, that choice
+  must be used throughout the tree.  In projects that have nested projects
+  in subdirectories, be sure to convert everything together.
+
+The ``OLD`` behavior for this policy is to place MSVC runtime library
+flags in the default :variable:`CMAKE_<LANG>_FLAGS_<CONFIG>` cache
+entries and ignore the :variable:`CMAKE_MSVC_RUNTIME_LIBRARY` abstraction.
+The ``NEW`` behavior for this policy is to *not* place MSVC runtime
+library flags in the default cache entries and use the abstraction instead.
+
+This policy was introduced in CMake version 3.15.  Use the
+:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
+Unlike many policies, CMake version |release| does *not* warn
+when this policy is not set and simply uses ``OLD`` behavior.
+
+.. include:: DEPRECATED.txt