Doxygen: Include documentation of define dependant code.

Generated documentation for unittests and other module was not
generated because doxygen was not assuming the define to be set.
This patch adds the necessary define for unittests and does the
same for NFQ, PFRING, IPFW, AF_PACKET and Prelude. This also adds
a variable that permit parallelisation of dot file generation.

doxygen.cfg

@@ -990,7 +990,7 @@ INCLUDE_FILE_PATTERNS  =
 # undefined via #undef or recursively expanded use the := operator 
 # instead of the = operator.
 
-PREDEFINED             = 
+PREDEFINED             = UNITTESTS NFQ IPFW HAVE_PFRING HAVE_AF_PACKET PRELUDE
 
 # 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. 
@@ -1228,3 +1228,6 @@ DOT_CLEANUP            = YES
 # used. If set to NO the values of all tags below this one will be ignored.
 
 SEARCHENGINE           = YES 
+
+# Parallel graph generation: 0 to auto detect
+DOT_NUM_THREADS = 0