Syntax: Line joining with &\

Syntax: Line joining with &\

make has a weird rule for line joining in recipes, where the \ is
retained.  This is very annoying espcially when writing little scripts
in other languages in recipes.  The GNU make documentation suggests
using a `define' block, in which the \ are still necessary, but
vanish.  But that separates out the recipe from the rule, and also
demands that the recipe text be given a name.

(Note that .ONESHELL offers an alternative but sadly is not available
on a per-rule basis.  This is even less sensible with non-recursive
make, where you might want different directories to have different
settings.)

So, provide &\ which does the other thing, swallowing the newline and
discarding the &\, even in rules.

Add a test case too.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>