* Added cxxstd value 20 for C++20.
The -std=c++20 is supported by clang 10.
* Updated cxxstd=latest for clang 5.0 and newer.
clang 5.0 supports -std=c++17. clang 6.0 supports -std=c++2a.
clang 10.0 supports -std=c++20.
* Updated cxxstd=latest for gcc.
gcc 4.9 supports -std=c++14. gcc 6 supports -std=c++17. gcc 10 will support
-std=c++20.
* Added support for cxxstd=20 to toolsets that don't support C++20.
local-visibility is intended to be used by libraries or targets that require
a particular visibility mode. It is not propagated to dependencies. It is
equivalent to the previous visibility feature.
The new visibility feature is a composite propagated feature, so it can be
specified by users and higher level targets as a requirement. This feature is
translated to local-visibility.
The new visibility feature can be used to specify default symbol visibility
on compilers and platforms that support it. The default visibility is
global, which matches most compilers' defaults. In gcc documentation it is
called the "default" visibility. Other modes are: protected and hidden.
