Fix typo in script/dropbox_upload.py
Add EXCLUDE_PATTERNS to src/Doxyfile

- */src/cmake/*
- GTAGS GPATH GRTAGS
- memcheck.supp
- *.cmake *.cmake.in
-  *.txt
Use GNU parallel to upload doxygen files to dropbox

[skip ci]
Add partial support for metavariables in the normalizer

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>