I believe it is becasue of this:
* WARNING: Backward-incompatibility!
In order to comply with POSIX, the way in which GNU
backslash-newline sequences in command strings has
changed. If your
makefiles use backslash-newline sequences inside of
strings in command scripts you will be impacted by
this change. See
the GNU make manual subsection "Splitting Command
"Splitting Lines"), in section "Command Syntax",
chapter "Writing the
Commands in Rules", for details.