Do not require all compilers available on manifest

Closes #4228

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: José Valim

lib/mix/lib/mix/tasks/compile.ex

@@ -127,8 +127,8 @@ defmodule Mix.Tasks.Compile do
   """
   def manifests do
     Enum.flat_map(compilers(), fn(compiler) ->
-      module = Mix.Task.get!("compile.#{compiler}")
-      if function_exported?(module, :manifests, 0) do
+      module = Mix.Task.get("compile.#{compiler}")
+      if module && function_exported?(module, :manifests, 0) do
         module.manifests
       else
         []

lib/mix/test/mix/tasks/compile_test.exs

@@ -26,6 +26,12 @@ defmodule Mix.Tasks.CompileTest do
     assert_received {:mix_shell, :info, ["\nEnabled compilers: elixir, app, custom, protocols"]}
   end
 
+  test "compile does not require all compilers available on manifest" do
+    Mix.Project.push CustomCompilers
+    assert Mix.Tasks.Compile.manifests |> Enum.map(&Path.basename/1) ==
+           [".compile.elixir"]
+  end
+
   test "compile a project with mixfile" do
     in_fixture "no_mixfile", fn ->
       assert Mix.Tasks.Compile.run([]) == :ok