build(cmake): Force include CppMacros.h globally for VC6

[abhatem]

Many source files in Generals and GeneralsMD use modern C++ features like 'nullptr' (polyfilled in CppMacros.h) without explicitly including the header. This caused build failures on VC6 (Fixes #2165).

This change:

1. Adds 'Dependencies/Utility' to the global include path (guarded by IS_VS6_BUILD).
2. Uses the MSVC '/FI' flag to force include 'Utility/CppMacros.h' when compiling with VC6. This ensures compatibility macros like 'nullptr' (defined as 0) are always available.

[greptile-apps]

Greptile Overview

Greptile Summary

This PR fixes VC6 build failures by forcing inclusion of CppMacros.h, which provides C++11 compatibility polyfills like nullptr (defined as 0 for older compilers). Many source files use these modern features without explicitly including the header.

• Added Dependencies/Utility to global include path for VC6 builds
• Used MSVC /FI flag to force include Utility/CppMacros.h in all compilation units
• Changes are properly guarded with IS_VS6_BUILD condition
• Includes comment indicating this is temporary until VC6 support is dropped

The implementation is clean, follows established patterns in the codebase, and solves the build issue without requiring modifications to hundreds of source files.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The change uses standard MSVC compiler flags, is properly guarded by IS_VS6_BUILD conditional, and follows established patterns in the codebase. The forced include approach is a well-known workaround that doesn't affect other build configurations.
• No files require special attention

Important Files Changed

Filename	Overview
CMakeLists.txt	Added forced include of CppMacros.h for VC6 builds to provide C++11 polyfills like nullptr

Sequence Diagram

sequenceDiagram
    participant CMake
    participant Compiler as MSVC VC6
    participant Sources as Source Files
    participant CppMacros as CppMacros.h
    
    CMake->>CMake: Check IS_VS6_BUILD
    alt IS_VS6_BUILD is TRUE
        CMake->>CMake: Add Dependencies/Utility to include paths
        CMake->>Compiler: Set /FIUtility/CppMacros.h flag
        loop For each source file
            Compiler->>CppMacros: Force include at start of compilation
            CppMacros->>Compiler: Define nullptr as 0
            CppMacros->>Compiler: Define other C++11 polyfills
            Compiler->>Sources: Compile with polyfills available
        end
    else IS_VS6_BUILD is FALSE
        Compiler->>Sources: Compile normally (C++11 support)
    end

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[greptile-apps]

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[xezon]

Ok interesting. Thanks for fixing.

[xezon]

Can we omit ${CMAKE_SOURCE_DIR} or is that strictly required?

[abhatem]

true, it's not needed, it's a remnant from when I was experimenting 😅

Also after giving it a second look today, I didn't like the generator expression I had: it had a check for if it's a MSVC compiler ($<CXX_COMPILER_ID:MSVC>:) while it's enclosed in a if (IS_VS6_BUILD) and cmake generators are ugly imo. I rewrote it to a more readable line now.