cmake: fix warnings in external projects code

With 3.31:
```
Keyword WORKING_DIRECTORY requires a value, but none was given.

  Policy CMP0175 is not set: add_custom_command() rejects invalid arguments.
  Run "cmake --help-policy CMP0175" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning
  Keyword WORKING_DIRECTORY requires a value, but none was given.
```

Additionally, make calls to `add_custom_command` stricter starting with 3.31.

thirdparty/cmake_modules/koreader_external_project.cmake

@@ -5,6 +5,11 @@ if(POLICY CMP0116)
     cmake_policy(SET CMP0116 NEW)
 endif()
 
+# Stricter `add_custom_command()` (rejects invalid argument(s)).
+if(POLICY CMP0175)
+    cmake_policy(SET CMP0175 NEW)
+endif()
+
 # Crappy macOS command line utilities strike again…
 set(PRINTF_QS "%q")
 if(CMAKE_HOST_APPLE)
@@ -65,8 +70,10 @@ function(external_project_step NAME)
             DEPENDS $<TARGET_PROPERTY:${PROJECT_NAME}-${NAME},DEPENDS>
             VERBATIM
             ${_USES_TERMINAL}
-            WORKING_DIRECTORY ${_WORKING_DIRECTORY}
         )
+        if(DEFINED _WORKING_DIRECTORY)
+            list(APPEND SPEC WORKING_DIRECTORY ${_WORKING_DIRECTORY})
+        endif()
         if(DEFINED _DEPFILE)
             list(APPEND SPEC DEPFILE ${_DEPFILE})
         endif()