CMakeLists.txt proposed fixes for building on Gentoo Linux

[Anoncheg1]

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

https://github.com/Anoncheg1/anonch-overlay/blob/main/dev-games/tempest/tempest-20251214.ebuild

it is for 2025-12-14 58477f3

In this ebuild I use system Gentoo packages, instead of built-in, with partial success (for libsquish built-in include headers .h still used)

• media-libs/libpng
• media-libs/libsquish
• sys-libs/zlib

I tested it with working OpenGothic

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

Suggested improvements for CMakeLists.txt file:

• Use GNUInstallDirs for Installation Paths
  Replace hardcoded lib and include destinations in the install() directive with ${CMAKE_INSTALL_LIBDIR} and ${CMAKE_INSTALL_INCLUDEDIR}. This ensures correct multilib directory handling (e.g., installing to lib64 on Gentoo/Arch systems) without requiring downstream patches.

• Remove Hardcoded macOS Tool Paths
  Change find_program(GLSLANGVALIDATOR glslangValidator "/opt/homebrew/bin") to find_program(GLSLANGVALIDATOR glslangValidator REQUIRED). This allows the system's standard PATH resolution to work correctly on all platforms, rather than forcing a macOS-specific fallback that can cause confusion or failures on Linux.

• Adopt Standard CMake Vulkan Find Module
  Replace manual $ENV{VULKAN_SDK} path injections and direct vulkan / vulkan-1 linking with the standard find_package(Vulkan REQUIRED) and the modern imported target Vulkan::Vulkan. This is more robust, cross-platform, and respects system-wide Vulkan SDK installations managed by distribution package managers.

• Add Options for System Third-Party Libraries
  Introduce CMake options (e.g., TEMPEST_USE_SYSTEM_ZLIB, TEMPEST_USE_SYSTEM_LIBPNG, TEMPEST_USE_SYSTEM_SQUISH). When enabled, these should use find_package() and expose the expected target names (zlibstatic, png_static, squish-tempest) as INTERFACE libraries.
  Why this matters: Downstream packagers prefer using system libraries for security and deduplication. By providing INTERFACE targets with the same names, internal target_link_libraries calls remain intact, and we avoid CMake errors caused by applying PRIVATE compile options to system/INTERFACE targets.

• Fix Relative Includes in Public Headers
  Update public headers in include/Tempest/ that currently use #include "../" to use #include "./" or #include "Tempest/". When headers are installed to a system directory (e.g., /usr/include/Tempest/), a ../ relative path points outside the valid include root, causing compilation failures for downstream users.

Benefits to the Project:

• 📦 Easier Packaging: Distributions can package Tempest without maintaining fragile, version-breaking patch files.
• 🛡️ Security & Stability: Encourages the use of system-managed, patched, and audited third-party dependencies (zlib, libpng, squish).
• 🧹 Cleaner CMake: Aligns the project with modern CMake best practices (imported targets, GNUInstallDirs, proper find_package usage), making the codebase easier to maintain.