Libbacktrace: Fix the use of newline in sed replacement
On non-ELF targets, the Makefile needs a newline inside the sed REPLACE
string. The way it is currently done fails with GNU Make < 4, but GCC
only requires "GNU make version 3.80 (or later)".
The portable solution is given in the autoconf manual:
https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Newlines-in-Make-Rules.html