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.

Added a workaround for uncaught_exceptions for older Mac OS and iOS.

libc++ disables std::uncaught_exceptions for Mac OS < 10.12 and
iOS < 10.0, even though it defines __cpp_lib_uncaught_exceptions.

Fixes boostorg#80.

Untabify lightweight_test_with_fail

Use BOOST_INLINE_CONSTEXPR in empty_value

Go back to ::quick_exit, but include the correct <stdlib.h>; on Linux…

… and Cygwin, quick_exit is not brought into std in C++03 mode

Add clang-9 to Travis

Add clang-9 to Travis

Only return an 8 bit result from report_errors

Only return an 8 bit result from report_errors