Summary
Update Git-based documentation to reflect the compilers, tools and language standard versions that WiredTiger and its associated test code and utilities requires.

Wiki-based documentation will be updated separately as required (ie without a ticket).

Motivation

• Does this affect any team outside of WT?
  Not directly, but it will make it easier for other teams to build and use WiredTiger in the future.

• Is this issue urgent?
  Not urgent.

Acceptance Criteria (Definition of Done)

This ticket will be done when all documentation updates (including online documentation and readme.md files) have been updated.

Note: no architecture guide changes are required for this ticket.

Changes required:

• build-posix.dox - update text to explain the required language standard and tool versions.
• cmake/README.md - update text to explain the required language standard and tool versions.
• check and update any other README.md files as required.