Exclude 3rd party code from generated docs

Doxyfile.in

@@ -778,7 +778,7 @@ RECURSIVE              = YES
 # Note that relative paths are relative to the directory from which doxygen is
 # run.
 
-EXCLUDE                =
+EXCLUDE                = @CMAKE_CURRENT_SOURCE_DIR@/src/3rd_party
 
 # The EXCLUDE_SYMLINKS tag can be used to select whether or not files or
 # directories that are symbolic links (a Unix file system feature) are excluded