Aegisub/aegisub/build/fftw/config.xml

<?xml version="1.0" encoding="utf-8"?>
<Rule Name="ExternalSourceLocation"
    DisplayName="Source Location"
    Order="1"
    PageTemplate="tool"
    xmlns="http://schemas.microsoft.com/build/2009/properties">

  <Rule.DataSource>
    <DataSource Persistence="UserFile" ItemType="" HasConfigurationCondition="false" />
  </Rule.DataSource>

  <StringProperty
    Subtype="folder"
    Name="FftwSrcDir"
    DisplayName="Source Location"
    Description="A directory containing a copy of FFTW3, or empty to disable FFTW3. Changes to this property do not take effect until the project is reloaded."
    />
</Rule>