Update revision history

Update revision history

Updated check for apt-add-repository capabilities.

In Ubuntu 20.04 there appeared an updated version of the
software-properties-common package in focal-updates, which ships a newer
apt-add-repository version that doesn't support -P/-S/-U command line arguments.

Since we cannot rely on package version checks to determine apt-add-repository
capabilities, we have to parse its --help output instead.

Also, made source list processing more protected against spaces.

MSVC warning workaround for empty_value

Merge pull request boostorg#93 from sdarwin/feature/gha_fix

Small GHA fix, create destination dir

Changed quotes in format() expression.

Update copyright comments

Update copyright comments

Switch to from c++2a to c++20 for gcc-10 and clang-10.

Switch to from c++2a to c++20 for gcc-10 and clang-10.