makefile-cleanup-continuations

makefile-cleanup-continuations is a variable defined in `make-mode.el'.
Its value is

nil

Documentation:
If non-nil, automatically clean up continuation lines when saving.
A line is cleaned up by removing all whitespace following a trailing
backslash. This is done silently.
IMPORTANT: Please note that enabling this option causes Makefile mode
to MODIFY A FILE WITHOUT YOUR CONFIRMATION when "it seems necessary".

You can customize this variable.