hints/{linux,macOS}.2020

hints/{linux,macOS}.2020

A recent change was intended to allow specifying
  make CFLAGS=-O
on the command line to override our default of -g, but it didn't
work as intended.  foo=bar and foo+=bar don't work if foo has
been given a value on the command line.  The first was expected
behavior but the second wasn't, at least for me.  GNU make allows
'override foo+=bar' to cope with that.  (We're already implicitly
requiring GNU make for the linux and OSX hints.)