macOS: prefer package-manager SDL2 over a stale system framework

[quarrel07]

Problem

On macOS, a stale or partial /Library/Frameworks/SDL2.framework (left behind by some installers) shadows a working Homebrew/MacPorts SDL2 and breaks configuration:

File or directory /Library/Headers referenced by variable SDL2_INCLUDE_DIR does not exist !

The framework's bundled CMake config sets SDL2_INCLUDE_DIR to a non-existent /Library/Headers, so find_package(SDL2 REQUIRED) fails even though a perfectly good SDL2 is installed via Homebrew. The only current workaround is to pass -DSDL2_DIR=... (or delete the system framework) manually.

Fix

In cmake/dependencies/mac.cmake, search frameworks last and add the Homebrew prefix to the search path, so the package-manager SDL2 (and its CMake config) is preferred automatically:

set(CMAKE_FIND_FRAMEWORK LAST)
find_program(BREW_EXECUTABLE brew)
if (BREW_EXECUTABLE)
    execute_process(COMMAND ${BREW_EXECUTABLE} --prefix
                    OUTPUT_VARIABLE BREW_PREFIX OUTPUT_STRIP_TRAILING_WHITESPACE)
    if (BREW_PREFIX)
        list(APPEND CMAKE_PREFIX_PATH "${BREW_PREFIX}")
    endif()
endif()

Scoped to the macOS dependency path; no effect on other platforms. One file changed; branched off current main.