Error if MKL requested for non-MKL Julia

deps/build.jl

@@ -2,11 +2,14 @@
 # In that case, we have to do this little dance to get around having to use BinDeps
 # for a library that's already linked to Julia.
 settings = joinpath(@__DIR__, "..", ".build_settings")
+action = ""
 if isfile(settings)
     provider = readchomp(settings)
+    action = "delete " * normpath(settings)
 elseif haskey(ENV, "JULIA_FFTW_PROVIDER")
     provider = ENV["JULIA_FFTW_PROVIDER"]
     open(f -> println(f, provider), settings, "w")
+    action = "unset the JULIA_FFTW_PROVIDER environment variable"
 else
     provider = ""
 end
@@ -25,6 +28,9 @@ if provider == "MKL" && Base.BLAS.vendor() === :mkl
             const libfftwf = "$mklpath"
         """)
     end
+elseif provider == "MKL"
+    error("MKL build requested for FFTW but Julia was not built with MKL.\n",
+          "To fix this, " action, " and rerun Pkg.build(\"FFTW\").")
 else
     include("build_fftw.jl")
 end