get the warning flags on linux and macOS into better alignment
move the compiler flags into hints/include/compiler.2020 so that
maintenance changes can be made there and take effect for both
linux and macOS
this will likely trigger some initial new code warnings to be
investigated on either or both platforms
Allow sharing of common code between different hints files
through use of: #-INCLUDE
new folder created: sys/unix/hints/include
new hints include files:
sys/unix/hints/include/multiw-1.2020
sys/unix/hints/include/multiw-2.2020
structure the early parts of sys/unix/hints/linux.2020 and
sys/unix/hints/macOS.2020 consistently, and utilize #-INCLUDE multiw-1.2020
and #-INCLUDE multiw-2.2020 in them. That will allow the Makefile lines
that they contain to be maintained in a single place.
