dev-haskell/cabal-install: fix build failure on ghc-7.6.3

(it's nonthreaded variant provides getNumberOfProcessors')
reported by mjrosenb:
    /usr/lib64/ghc-7.6.3/libHSrts.a(OSThreads.o):(.opd+0x18): multiple definition of `getNumberOfProcessors'

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>