Add an environment variable to disable CUDA usage.

src/Flux.jl

@@ -20,9 +20,18 @@ export SGD, Descent, ADAM, Momentum, Nesterov, RMSProp,
   ADAGrad, AdaMax, ADADelta, AMSGrad, NADAM,
   ADAMW, RADAM, InvDecay, ExpDecay, WeightDecay
 
+
+allow_cuda() = parse(Bool, get(ENV, "FLUX_USE_CUDA", "true"))
+const consider_cuda = allow_cuda()
+
 using CUDAapi
-if has_cuda()
-  using CuArrays
+if consider_cuda && has_cuda()
+  try
+    using CuArrays
+  catch
+    @error "CUDA is installed, but CuArrays.jl fails to load. Please fix the issue, or load Flux with FLUX_USE_CUDA=false."
+    rethrow()
+  end
   use_cuda() = true
 else
   use_cuda() = false
@@ -47,7 +56,9 @@ if use_cuda()
 end
 
 function __init__()
-  if has_cuda() != use_cuda()
+  # check if the GPU usage conditions that are baked in the precompilation image
+  # match the current situation, and force a recompilation if not.
+  if (allow_cuda() != consider_cuda) || (consider_cuda && has_cuda() != use_cuda())
       cachefile = if VERSION >= v"1.3-"
           Base.compilecache_path(Base.PkgId(Flux))
       else