Make build more parallelizable.
Make build more parallelizable.

It looks like it's possible to build the custom Xamarin targets
(monotouch/monodroid/xamarinmacmobile/xamarinmacfull) in parallel,
so modify the Makefile to support that.

This makes a joint monotouch+xamarinmac build ~40s faster
(from 2:30 to 1:50 on my machine).

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