From the Makefile format, I think this should not be leading whitespace, but only leading tabs. Leading spaces in rules are a syntax error. Can you check whether \t instead of \s also works?
Good catch, Arne. Will check.
Looks like the 10th duplicate of Bug#17400.
Also good catch, Mauro. Can someone merge these 2 bugs?
But honestly: A more than 10 year old bug!!! About time we get it fixed!