Hi,

There was a change today in gnulib, that requires a small change in
packages that use gnulib-tool --with-tests with --makefile-name.
GNU diffutils is one such package.

Currently, './bootstrap' fails like this:
...
autoreconf: running: automake --add-missing --copy --force-missing
gnulib-tests/gnulib.mk:47: error: AM_CFLAGS must be set with '=' before using '+='
gnulib-tests/Makefile.am:1:   'gnulib-tests/gnulib.mk' included from here
autoreconf: error: automake failed with exit status: 1
./bootstrap: autoreconf failed

The attached proposed patch fixes it.