In order to determine what character-set (Unicode, Multi-Byte, none)
shall be set in the generated `*.vcxproj` files, CMake checks if one of
the macros `_UNICODE` or `_SBCS` are defined.
However, as these macros can be defined with or without a value, the
check should always recognize these macros whether they are defined with
a value or without. That is now assured by this commit.

Fixes: #25379