in m4's patsubst, how do I replace newlines with spaces?

How can I tell m4's patsubstr to replace all newlines in a string with a space?

I've tried:

patsubst(MULTI_LINE_STR_DEFINE,`\n',` ')

and

p开发者_如何学Goatsubst(MULTI_LINE_STR_DEFINE,`\\n',` ')

patsubst(MULTI_LINE_STR_DEFINE,`
',` ')

That is, you put the newline, literally, between quotes.