Upgrade to 3.9.0 release.
Improve support for -fopenmp with a hack inspired by a submission from
Johannes Dieterich <dieterich@ogolem.org>.
Implement a number of improvments submitted by lightside@gmx.com:
- Add license information
- Use VAR option helper for _USES_PYTHON variable
- Add libxml2 to USE_GNOME for CLANG option
- Sort variables for options (e.g. GOLD_*)
Add USES=execinfo for LLDB.
PR: 203223, 212334