Minimum Make version supported.
Anything older either breaks bmakelib or can cause it to behave in unexpected ways.
Expands to a single newline (\n)
Expands to a comma (,)
Expands to a slash (/)
Expands to a backslash ()
Expands to a whitespace
Expands to an empty string.
This silly-looking variable is a formatting convenience for cases when whitespace is significant
and will affect the evaluation. See bmakelib.shell.error-if-nonzero for an example.