Support defines without value (e.g. #define PDF_USE_XFA)

[mara004]

pdfium has this construct:

#ifdef PDF_ENABLE_XFA
// PDF_USE_XFA is set in confirmation that this version of PDFium can support
// XFA forms as requested by the PDF_ENABLE_XFA setting.
#define PDF_USE_XFA
#endif  // PDF_ENABLE_XFA

When passing --define PDF_ENABLE_XFA to ctypesgen, members guarded by #ifdef PDF_ENABLE_XFA correctly appear in the output file, except PDF_USE_XFA. It seems like defines without value are not supported by ctypesgen?

In this case it should be translated to PDF_USE_XFA = True in python.