build(cmake): add prep/init.cmake (#2375)

Co-authored-by: James Le Cuirot <chewi@aura-online.co.uk>

cmake/prep/init.cmake

@@ -0,0 +1,9 @@
+if (WIN32)
+elseif (APPLE)
+elseif (UNIX)
+    include(GNUInstallDirs)
+
+    if(NOT DEFINED SUNSHINE_EXECUTABLE_PATH)
+        set(SUNSHINE_EXECUTABLE_PATH "sunshine")
+    endif()
+endif ()

cmake/prep/special_package_configuration.cmake

@@ -6,8 +6,6 @@ if (APPLE)
         configure_file(packaging/macos/sunshine.rb sunshine.rb @ONLY)
     endif()
 elseif (UNIX)
-    include(GNUInstallDirs)  # this needs to be included prior to configuring the desktop files
-
     # configure the .desktop file
     if(${SUNSHINE_BUILD_APPIMAGE})
         configure_file(packaging/linux/AppImage/sunshine.desktop sunshine.desktop @ONLY)