Replace our hard-coded default for `/RTC1` with a first-class
abstraction to select runtime checks from an enumeration of logical
names. Add a `MSVC_RUNTIME_CHECKS` target property and corresponding
`CMAKE_MSVC_RUNTIME_CHECKS` variable.
Removing the old default flag requires a policy because existing
projects may rely on string processing to edit them and choose
runtime checks under the old behavior. Add policy CMP0184 to
provide compatibility.
Fixes: #26614
!3211 overlooked populating the runtime library selection flags for
clang-cl in MSVC compatibility mode. There is no flag that needs to be
passed, but the value is expected to be available by the generators. We
simply provide the empty string to appease the generators without
emitting any additional flags.
Fixes: #23048
This allows building .s/.S assembly code, and makes sure that the
`-fPIC` option isn't passed to the compiler (as it errors out on
Clang and causes a loud warning on GCC).
AJIOB