make -j doesn't work in libraries/ on Cygwin

This prevents validate from working on Windows at the moment:

[camresppta01] /cygdrive/c/builds/ghc-HEAD/libraries > make -j2
ifBuildable/ifBuildable base make
make[1]: Entering directory `/cygdrive/c/builds/ghc-HEAD/libraries/base'
make[1]: *** read jobs pipe: Is a directory.  Stop.
make[1]: *** Waiting for unfinished jobs....
make[1]: Leaving directory `/cygdrive/c/builds/ghc-HEAD/libraries/base'
make: *** [make.library.base] Error 2

Trac metadata

Trac field	Value
Version	6.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Build System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture	Unknown