count newlines as spaces for tokenizing backtick-interpolated prerequisites

[Daphne Preston-Kendal]

Prerequisites of this form:

task:V: `find dir -name \*.src | sed 's/\.src$/.bin/'`

will only ever build the first prerequisite returned because the rest are separated by newlines instead of spaces.

A quick fix is to put something like | perl -pe 's/\n/ /g' at the end of the shell command.