CMakeLists.txt: don't pass header args as single space-separated string

let python's parse command-line args as usual

CMakeLists.txt

@@ -834,9 +834,8 @@ set_target_properties(hb-ot-tag PROPERTIES COMPILE_FLAGS "-DMAIN")
 if (UNIX OR MINGW)
   if (BUILD_SHARED_LIBS)
     # generate harfbuzz.def after build completion
-    string(REPLACE ";" " " space_separated_headers "${project_headers}")
     add_custom_command(TARGET harfbuzz POST_BUILD
-      COMMAND "${PYTHON_EXECUTABLE}" ${PROJECT_SOURCE_DIR}/src/gen-def.py ${PROJECT_BINARY_DIR}/harfbuzz.def ${space_separated_headers}
+      COMMAND "${PYTHON_EXECUTABLE}" ${PROJECT_SOURCE_DIR}/src/gen-def.py ${PROJECT_BINARY_DIR}/harfbuzz.def ${project_headers}
       WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}/src)
 
     add_test(NAME check-static-inits.sh