When targeting the MSVC ABI, define `_MBCS` by default if the project
does not define `_SBCS` or `_UNICODE`. Visual Studio has long defined
one of the three character set macros automatically. For consistency,
define it when compiling for the MSVC ABI with other generators.
Add policy CMP0204 for compatibility.
Fixes: #27275
In a per-component installation the generated installation scripts
are invoked once for each component.
Per default custom installation script code added by install(CODE|SCRIPT)
only runs for one specific component in this context.
The new ALL_COMPONENTS option allows custom script code to be run once
for each component being installed.
