We should not be seeing this problem at all on Darwin, as lib/Makefile.am's 
include directive is protected inside an 'if BUILDING_FOR_WINDOWSNT'.... Oh, I 
see, the problem is that we are using a Automake conditional -- these days we 
assume GNU make so this should just be an ordinary GNU make conditional, which 
would have avoided the problem.

Anyway, I installed the attached patch to work around the immediate issue; 
please give it a try.