Verify minimum required version of sphinx-build
Verify minimum required version of sphinx-build

CentOS 6.6 includes sphinx-build 0.6.6 which is woefully inadequate to
build the users guide. In particular it fails as it lacks
`sphinx.ext.extlinks`, which was introduced in 1.0.0. Looking at the
changelog it appears that 1.0.0 ought to work.

nushio3/ghc,oldmanmike/ghc,tjakway/ghcjvm,mcschroeder/ghc,shlevy/ghc,oldmanmike/ghc,tjakway/ghcjvm,mcschroeder/ghc,gridaphobe/ghc,elieux/ghc,GaloisInc/halvm-ghc,sdiehl/ghc,mettekou/ghc,tjakway/ghcjvm,siddhanathan/ghc,oldmanmike/ghc,mcschroeder/ghc,sdiehl/ghc,olsner/ghc,oldmanmike/ghc,sdiehl/ghc,vikraman/ghc,AlexanderPankiv/ghc,oldmanmike/ghc,mcschroeder/ghc,mcschroeder/ghc,snoyberg/ghc,tjakway/ghcjvm,olsner/ghc,ezyang/ghc,oldmanmike/ghc,vikraman/ghc,mettekou/ghc,elieux/ghc,elieux/ghc,snoyberg/ghc,shlevy/ghc,da-x/ghc,tjakway/ghcjvm,nushio3/ghc,vTurbine/ghc,da-x/ghc,siddhanathan/ghc,vikraman/ghc,AlexanderPankiv/ghc,vikraman/ghc,vTurbine/ghc,mcschroeder/ghc,siddhanathan/ghc,olsner/ghc,elieux/ghc,sdiehl/ghc,siddhanathan/ghc,GaloisInc/halvm-ghc,tjakway/ghcjvm,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,siddhanathan/ghc,sdiehl/ghc,gridaphobe/ghc,vTurbine/ghc,elieux/ghc,sgillespie/ghc,elieux/ghc,nushio3/ghc,da-x/ghc,da-x/ghc,vTurbine/ghc,sgillespie/ghc,nushio3/ghc,AlexanderPankiv/ghc,olsner/ghc,nushio3/ghc,olsner/ghc,snoyberg/ghc,AlexanderPankiv/ghc,gridaphobe/ghc,mettekou/ghc,shlevy/ghc,siddhanathan/ghc,gridaphobe/ghc,mettekou/ghc,shlevy/ghc,mcschroeder/ghc,GaloisInc/halvm-ghc,ezyang/ghc,sgillespie/ghc,vikraman/ghc,AlexanderPankiv/ghc,nushio3/ghc,vTurbine/ghc,da-x/ghc,AlexanderPankiv/ghc,sdiehl/ghc,ezyang/ghc,AlexanderPankiv/ghc,nushio3/ghc,ezyang/ghc,snoyberg/ghc,shlevy/ghc,siddhanathan/ghc,GaloisInc/halvm-ghc,sdiehl/ghc,gridaphobe/ghc,gridaphobe/ghc,shlevy/ghc,tjakway/ghcjvm,vikraman/ghc,ezyang/ghc,olsner/ghc,gridaphobe/ghc,ezyang/ghc,da-x/ghc,vTurbine/ghc,elieux/ghc,sgillespie/ghc,oldmanmike/ghc,da-x/ghc,shlevy/ghc,sgillespie/ghc,snoyberg/ghc,mettekou/ghc,snoyberg/ghc,vTurbine/ghc,vikraman/ghc,olsner/ghc,snoyberg/ghc,GaloisInc/halvm-ghc,mettekou/ghc,ezyang/ghc,sgillespie/ghc,sgillespie/ghc,mettekou/ghc