Can I define an environment variable and use it in conditional compilation?

Question

I know that I can do this in a *.h file:

#ifdef _DEBUG
#pragma comment(lib, \"libtiffd.lib\")
#else
#pragma comment(lib, \"libtiff.lib\")
#endif

Answer 1

My test: Create environment variable MY_VERSION = V2_4_6. Start VS, in project properties, C++, Preprocessor, Preprocessor Definitions, add $(MY_VERSION). This program:

#ifdef V2_4_6
    cout << "OK" << endl;
#else
    cout << "??" << endl;
#endif

prints "OK". Exit Visual Studio, change MY_VERSION value to another value or remove it. Start VS, rebuild the program. Now it prints "??".

Note that after changing the variable value it is neccesary to restart Visual Studio (since environment variables are not refreshed dynamically), and make Rebuild All.