doc: Make sure we have the defines to build all docs.

doc/doxy.config.in

@@ -1628,7 +1628,7 @@ INCLUDE_FILE_PATTERNS  =
 # undefined via #undef or recursively expanded use the := operator
 # instead of the = operator.
 
-PREDEFINED             =
+PREDEFINED             = WITH_SERVER WITH_SFTP WITH_PCAP
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then
 # this tag can be used to specify a list of macro names that should be expanded.