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12.12 Newlines in Make Rules
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In shell scripts, newlines can be used inside string literals.  But in
the shell statements of ‘Makefile’ rules, this is not possible: A
newline not preceded by a backslash is a separator between shell
statements.  Whereas a newline that is preceded by a backslash becomes
part of the shell statement according to POSIX, but gets replaced,
together with the backslash that precedes it, by a space in GNU ‘make’
3.80 and older.  So, how can a newline be used in a string literal?

   The trick is to set up a shell variable that contains a newline:

     nlinit=`echo 'nl="'; echo '"'`; eval "$$nlinit"

   For example, in order to create a multi-line ‘sed’ expression that
inserts a blank line after every line of a file, this code can be used:

     nlinit=`echo 'nl="'; echo '"'`; eval "$$nlinit"; \
     sed -e "s/\$$/\\$${nl}/" < input > output