On some versions of HP-UX, make reads multiple newlines
following a backslash, continuing to the next non-empty line. For
example,
FOO = one \
BAR = two
test:
: FOO is "$(FOO)"
: BAR is "$(BAR)"
shows FOO equal to one BAR = two. Other implementations
sensibly let a backslash continue only to the immediately following
line.