Enable warning Wundef

It is better to use "#ifdef" for testing macros instead of "#if"
In header file, "#ifdef DOXYGEN" was used 30 times and "#if DOXYGEN"
23 times. This patch makes it consistent and enable warning Wundef to prevent
this kind of issues.

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
7 files changed