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