CMakeLists.txt proposed fixes for building on Gentoo Linux

[Anoncheg1]

Hi! I created Ebuild file for Gentoo Linux with instruction to compile package from sources.

https://github.com/Anoncheg1/anonch-overlay/blob/main/games-engines/opengothic/opengothic-1.0.3549.ebuild

it is for Dec 23 2025 abf079e

Motivation

Linux distributions (e.g., Gentoo, Arch, NixOS) strongly prefer linking against system-installed libraries for security updates, binary deduplication, and easier maintenance. Currently, packaging OpenGothic requires maintainers to apply fragile sed patches to strip out bundled submodules, which is prone to breaking on upstream updates.

Changes

This PR introduces three new CMake options, all defaulting to OFF to preserve the exact existing behavior for end-users building from source:

• USE_SYSTEM_ZENKIT: Uses system zenkit and squish libraries.
• USE_SYSTEM_TEMPEST: Uses system Tempest and openal libraries.
• USE_SYSTEM_BULLET: Uses system bullet libraries via find_package(Bullet).

Benefits

1. Zero impact on default builds: Regular users cloning the repo and running cmake .. will experience no changes.
2. Clean packaging: Distributors can now simply pass -DUSE_SYSTEM_ZENKIT=ON -DUSE_SYSTEM_TEMPEST=ON -DUSE_SYSTEM_BULLET=ON to CMake, eliminating the need for ebuild/PKGBUILD patching.
3. Proper dependency chaining: The system library targets correctly declare their INTERFACE_LINK_LIBRARIES (e.g., system zenkit links to squish, system Tempest links to openal), preventing "DSO missing from command line" linker errors.
4. Code cleanup: Removed accidental empty if() blocks that were left behind in the Bullet section.

(Note: There is also a minor C++ deprecation warning in game/world/objects/interactive.cpp regarding inter.state. I recommend changing it to inter.state_count to ensure clean builds on modern compilers, but I have kept this PR strictly focused on CMake.)