Using -DCMAKE_CXX_FLAGS inhibits MSVC built-in defines

Expected Behavior

The built-in WIN32 value appears in vcxproj PreprocessorDefinitions entries regardless of whether -DCMAKE_CXX_FLAGS was specified.

Actual Behavior

When running CMake using a Visual Studio Generator (tested using "Visual Studio 17 2022") and specifying -DCMAKE_CXX_FLAGS on the commandline, the generated .vcxproj file loses built-in PreprocessorDefinitions entries, notably missing WIN32. When used with a project also using the rc compiler, notably the RcCompile section in the vcxproj retains the built-in defines (as expected since LANG specific flags are provided, I only mention it since it makes for a useful side-by-side comparison.

Conversely, if running without -DCMAKE_CXX_FLAGS then the generated .vcxproj file does include the full built-in PreprocessorDefinitions entries, including WIN32.

Oddly, if one instead sets the CXXFLAGS environment variable (indirectly initializing CMAKE_CXX_FLAGS) then the full built-in definitions are included.

Whether WIN32 should or shouldn't be provided by CMake or assumed by a project is a ship which has likely sailed long ago (likely mimicking new project creation from within Visual Studio?). However, this issue is that since it is provided as a built-in definition, specifying additional flags via CMAKE_CXX_FLAGS should not inhibit the addition.

Indirectly specifying via the CXXFLAGS environment variable is only a partial workaround since it doesn't permit changing CMAKE_CXX_FLAGS in a pre-existing build area.

This was tested to occur with 3.16, 3.22, and 3.23.