Remove verbose code to check if a subdir exists

The cmake build-in messages are good enough for us. No need to implement
custom things here.

CMakeLists.txt

@@ -218,17 +218,8 @@ if(APPLE)
   endif()
 endif()
 
-if(EXISTS "${CMAKE_HOME_DIRECTORY}/3rdparty/edbee-lib/CMakeLists.txt")
-  add_subdirectory(3rdparty/edbee-lib/edbee-lib)
-elseif()
-  message(FATAL_ERROR "Cannot locate edbee-lib editor widget submodule source code, build abandoned!")
-endif()
-
-if(EXISTS "${CMAKE_HOME_DIRECTORY}/3rdparty/qtkeychain/CMakeLists.txt")
-  add_subdirectory(3rdparty/qtkeychain)
-elseif()
-  message(FATAL_ERROR "Cannot locate QtKeychain submodule source code, build abandoned!")
-endif()
+add_subdirectory(3rdparty/edbee-lib/edbee-lib)
+add_subdirectory(3rdparty/qtkeychain)
 
 file(GLOB_RECURSE lua_files RELATIVE "${CMAKE_HOME_DIRECTORY}/src/mudlet-lua/lua/" "${CMAKE_HOME_DIRECTORY}/src/mudlet-lua/lua/*.lua")
 list(LENGTH lua_files lua_file_count)
@@ -269,26 +260,18 @@ elseif()
 endif()
 
 if(USE_UPDATER)
-  if(EXISTS "${CMAKE_HOME_DIRECTORY}/3rdparty/dblsqd/CMakeLists.txt")
-    add_subdirectory(3rdparty/dblsqd)
-  else()
-    message(FATAL_ERROR "Cannot locate DBLSQD updater submodule source code, build abandoned!")
-  endif()
+  add_subdirectory(3rdparty/dblsqd)
 
   if(APPLE)
-    if(EXISTS "${CMAKE_HOME_DIRECTORY}/3rdparty/sparkle-glue/CMakeLists.txt")
-      add_subdirectory(3rdparty/sparkle-glue)
-    else()
-      message(FATAL_ERROR "Cannot locate Sparkle glue for updater submodule source code, build abandoned!")
-    endif()
-
+    add_subdirectory(3rdparty/sparkle-glue)
     if(NOT EXISTS "${CMAKE_HOME_DIRECTORY}/3rdparty/cocoapods/Pods/Sparkle")
       message(STATUS "Sparkle CocoaPod is missing, running 'pod install' to get it...")
       execute_process(TIMEOUT 30
         WORKING_DIRECTORY "${CMAKE_HOME_DIRECTORY}/3rdparty/cocoapods"
         COMMAND pod install)
     endif()
   endif()
+
 endif()
 
 find_program(CCACHE_FOUND ccache)