refactor(cmake): split CMakeLists into modules (#1587)

cmake/prep/special_package_configuration.cmake

@@ -0,0 +1,40 @@
+if (APPLE)
+    if(${SUNSHINE_CONFIGURE_PORTFILE})
+        configure_file(packaging/macos/Portfile Portfile @ONLY)
+    endif()
+elseif (UNIX)
+    # configure the .desktop file
+    if(${SUNSHINE_BUILD_APPIMAGE})
+        configure_file(packaging/linux/AppImage/sunshine.desktop sunshine.desktop @ONLY)
+    elseif(${SUNSHINE_BUILD_FLATPAK})
+        configure_file(packaging/linux/flatpak/sunshine.desktop sunshine.desktop @ONLY)
+        configure_file(packaging/linux/flatpak/sunshine_kms.desktop sunshine_kms.desktop @ONLY)
+    else()
+        configure_file(packaging/linux/sunshine.desktop sunshine.desktop @ONLY)
+    endif()
+
+    # configure metadata file
+    configure_file(packaging/linux/sunshine.appdata.xml sunshine.appdata.xml @ONLY)
+
+    # configure service
+    configure_file(packaging/linux/sunshine.service.in sunshine.service @ONLY)
+
+    # configure the arch linux pkgbuild
+    if(${SUNSHINE_CONFIGURE_PKGBUILD})
+        configure_file(packaging/linux/Arch/PKGBUILD PKGBUILD @ONLY)
+    endif()
+
+    # configure the flatpak manifest
+    if(${SUNSHINE_CONFIGURE_FLATPAK_MAN})
+        configure_file(packaging/linux/flatpak/dev.lizardbyte.sunshine.yml dev.lizardbyte.sunshine.yml @ONLY)
+    endif()
+endif()
+
+# return if configure only is set
+if(${SUNSHINE_CONFIGURE_ONLY})
+    # message
+    message(STATUS "SUNSHINE_CONFIGURE_ONLY: ON, exiting...")
+    set(END_BUILD ON)
+else()
+    set(END_BUILD OFF)
+endif()