Make it possible to build empty libraries again.

[23Skidoo]

Restores old behaviour broken by @igfoo in 8395c5d. Fixes #1241.

[tibbe]

@igfoo might want to take a look as well but it's LGTM from what I can see.

[cartazio]

i'm trying to build this version of cabal-install, and I can't get the associated version of cabal library to build! (and this version of cabal-install requires 1.17.0 )

[cartazio]

carter cabal/cabal ‹master*› » cabal configure
Resolving dependencies...
cabal: The package requires Cabal library version 1.17.0 but no suitable
version is installed.

[23Skidoo]

@cartazio Try using cabal-dev (that's how I build it):

$ cd cabal/cabal-install
$ cabal-dev install --reinstall ../Cabal
$ cabal-dev install

Hope that helps.

[cartazio]

i can reproduce still having that build problem on my current machine... and i believe i have head installed etc. (though i'll double check)

diagrams/diagrams#2

[cartazio]

i'll try building my cabal-install from head again