Backslash handling not POSIX-compliant

[Ivan Kozlov]

Doubling the backslash suppresses special newline handling in GNU make. I don’t 
see anything in the standard that allows this. It defines an escaped newline as 
one preceded by a backslash and doesn’t say anything about backslashes being 
treated specially otherwise.

Test case:
>.POSIX:
>test:
>       echo \\
>       true

Expected result: the command line
>echo \\
>true
being run.

>\
>true
being written to the standard output.

Actual result: two command lines 
>echo \\
and
>true
being run.

>\
being written to the standard output.