doxygen: include extra information about extra CFLAGS

configure.ac

@@ -318,6 +318,10 @@ case $host in
 	AC_MSG_ERROR([==> If you can provide patches to support your operating system please write to 'libcoap-developers@lists.sourceforge.net'.])
 esac
 
+# Exporting the PREDEFINED_CFLAGS definition
+PREDEFINED_CFLAGS=`echo $ADDITIONAL_CFLAGS | sed -e 's/-D//g'`
+AC_SUBST(PREDEFINED_CFLAGS)
+
 # And finaly combining the CFLAGS together ...
 CFLAGS+=" $ADDITIONAL_CFLAGS"
 

doc/Doxyfile.in

@@ -1974,7 +1974,7 @@ INCLUDE_FILE_PATTERNS  =
 # recursively expanded use the := operator instead of the = operator.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-PREDEFINED             =
+PREDEFINED             = @PREDEFINED_CFLAGS@
 
 # 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. The