Adjust configuration message to give better guidance
Adjust configuration message to give better guidance

On mac this now reports the following more helpful message:

configure: WARNING: /usr/local/bin/mono not found. Prefix should
normally be set to the mono installation path. Please re-run with
   ./autogen.sh
--prefix=/Library/Frameworks/Mono.framework/Versions/3.0.1

bleis-tift/FSharp.Compiler.Service,Heather/fsharp,BillHally/FSharp.Compiler.Service,cloudRoutine/FSharp.Compiler.Service,fsharp/fsharp,enricosada/fsharp,Jand42/FSharp.Compiler.Service,ncave/FSharp.Compiler.Service,Jand42/FSharp.Compiler.Service,forki/fsharp,7sharp9/fsharp,sergey-tihon/FSharp.Compiler.Service,sergey-tihon/FSharp.Compiler.Service,rolfbjarne/fsharp,alexischr/fsharp,rneatherway/FSharp.Compiler.Service,jeroldhaas/FSharp.Compiler.Service,forki/FSharp.Compiler.Service,mafiya69/fsharp,dsyme/FSharp.Compiler.Service,dsyme/FSharp.Compiler.Service,matthid/FSharp.Compiler.Service,directhex/fsharp,eloraiby/fsharp,rojepp/FSharp.Compiler.Service,alexischr/fsharp,forki/FSharp.Compiler.Service,bleis-tift/FSharp.Compiler.Service,directhex/fsharp,wk-j/fsharp,sergey-tihon/FSharp.Compiler.Service,BillHally/FSharp.Compiler.Service,alfonsogarciacaro/FSharp.Compiler.Service,alfonsogarciacaro/FSharp.Compiler.Service,enricosada/fsharp,matthid/FSharp.Compiler.Service,0x53A/FSharp.Compiler.Service,vasily-kirichenko/FSharp.Compiler.Service,ncave/FSharp.Compiler.Service,mafiya69/fsharp,cloudRoutine/FSharp.Compiler.Service,rojepp/FSharp.Compiler.Service,0x53A/FSharp.Compiler.Service,7sharp9/fsharp,fsharp/fsharp,yever/FSharp.Compiler.Service,dsyme/FSharp.Compiler.Service,eloraiby/fsharp,forki/FSharp.Compiler.Service,vasily-kirichenko/FSharp.Compiler.Service,rojepp/FSharp.Compiler.Service,forki/fsharp,alfonsogarciacaro/FSharp.Compiler.Service,jeroldhaas/FSharp.Compiler.Service,alexischr/fsharp,Jand42/FSharp.Compiler.Service,yever/FSharp.Compiler.Service,cloudRoutine/FSharp.Compiler.Service,ncave/FSharp.Compiler.Service,rolfbjarne/fsharp,matthid/FSharp.Compiler.Service,fsharp/fsharp,Heather/fsharp,wk-j/fsharp,0x53A/FSharp.Compiler.Service,vasily-kirichenko/FSharp.Compiler.Service,In2TheWild/fsharp,rneatherway/FSharp.Compiler.Service,In2TheWild/fsharp