message: build: fix PCH stub files generation optimization
diff: @@ -97,8 +97,8 @@ MACRO(_PCH_WRITE_PCHDEP_CXX _targetName _include_file _dephelp)
  set(${_dephelp} "${CMAKE_CURRENT_BINARY_DIR}/${_targetName}_pch_dephelp.cxx")
  set(_content "")
  - if(EXISTS "${_dephelp}")
  - file(READ "${_dephelp}" _content)
  + if(EXISTS "${${_dephelp}}")
  + file(READ "${${_dephelp}}" _content)
  endif()
  set(_dummy_str
  "#include \"${_include_file}\"
  