Enhance CMake build system with packaging and multi-platform support

[cerwym]

Asset packaging specifically left out to reduce size

[Copilot]

Pull request overview

Refactors the project’s CMake build into modular platform/dependency/target/packaging components, updates CI to build via CMake (including cross-compiled MinGW), and adjusts the make-based packaging flow to support a CPack-driven archive step.

Changes:

• Modularize CMake build logic into build/cmake/modules/* (platform detection, deps download/linking, target definitions, CPack packaging).
• Update GitHub Actions workflows to build binaries with CMake/Ninja and produce release archives via CPack (using make pkg-assemble only for data staging).
• Remove legacy build scripts (linux.mk, old deps/CMakeLists.txt) and add a helper script (build-cmake.sh) for local CMake builds.

Reviewed changes

Copilot reviewed 14 out of 15 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
package.mk	Adds pkg-assemble and data-only staging list for CPack-based packaging.
linux.mk	Removes the legacy Linux make-based build script.
deps/CMakeLists.txt	Removes the legacy CMake deps-download/link approach under deps/.
CMakeLists.txt	Switches to modular CMake includes (Platforms/Dependencies/BuildTargets/Packaging) and cache overrides for build number/suffix.
build/cmake/toolchains/mingw32.cmake	Adjusts MinGW toolchain compiler names to non--posix variants.
build/cmake/modules/Platforms.cmake	Adds platform detection and platform-scoped compile definitions.
build/cmake/modules/Packaging.cmake	Introduces CPack configuration and install rules to package binaries + staged data.
build/cmake/modules/Helpers.cmake	Adds shared functions for warnings/link flags/system libs by platform.
build/cmake/modules/Dependencies.cmake	Implements per-platform dependency fetching/import/linking (build-dir isolated).
build/cmake/modules/BuildTargets.cmake	Defines keeperfx targets, source filtering, and per-target flags/linking.
build-cmake.sh	Adds a local build helper (optionally Dockerized) for Windows cross-build and native Linux.
.gitignore	Stops ignoring build/ (now tracked for CMake modules); ignores build-* instead.
.github/workflows/build-release-patch-unsigned.yml	Moves Windows packaging build to CMake + CPack flow; installs CMake/Ninja.
.github/workflows/build-prototype.yml	Moves both Windows and Linux prototype builds to CMake; updates artifact paths and deps.
.github/workflows/build-alpha-patch-unsigned.yml	Moves Windows alpha packaging build to CMake + CPack flow; installs CMake/Ninja.

Suppressed comments (1)

build/cmake/modules/Packaging.cmake:18

• The usage example builds the package target from a build directory, but the configure step uses -B out. The build directory should match.

#   4. Create the archive:
#        cmake --build build --target package

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