unrevert r63852 but keep SIGCHLD path disabled for win32

Reading win32/win32.c waitpid implementation, maybe waitpid(-1, ...)
on that platform will never conflict with mjit use of waitpid.

In any case, I've added WAITPID_USE_SIGCHLD macro to vm_core.h
so it can be easy for Linux/BSD users to test (hopefully!)
win32-compatible code.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63855 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/core/process/wait2_spec.rb

@@ -4,31 +4,37 @@ describe "Process.wait2" do
   before :all do
     # HACK: this kludge is temporarily necessary because some
     # misbehaving spec somewhere else does not clear processes
+    # Note: background processes are unavoidable with MJIT,
+    # but we shouldn't reap them from Ruby-space
     begin
       Process.wait(-1, Process::WNOHANG)
-      $stderr.puts "Leaked process before wait2 specs! Waiting for it"
+      without_feature :mjit do
+        $stderr.puts "Leaked process before wait2 specs! Waiting for it"
+      end
       leaked = Process.waitall
-      $stderr.puts "leaked before wait2 specs: #{leaked}"
+      $stderr.puts "leaked before wait2 specs: #{leaked}" unless leaked.empty?
+      with_feature :mjit do
+        # Ruby-space should not see PIDs used by mjit
+        leaked.should be_empty
+      end
     rescue Errno::ECHILD # No child processes
     rescue NotImplementedError
     end
   end
 
-  without_feature :mjit do # [Bug #14867]
-    platform_is_not :windows do
-      it "returns the pid and status of child process" do
-        pidf = Process.fork { Process.exit! 99 }
-        results = Process.wait2
-        results.size.should == 2
-        pidw, status = results
-        pidf.should == pidw
-        status.exitstatus.should == 99
-      end
-    end
-
-    it "raises a StandardError if no child processes exist" do
-      lambda { Process.wait2 }.should raise_error(Errno::ECHILD)
-      lambda { Process.wait2 }.should raise_error(StandardError)
+  platform_is_not :windows do
+    it "returns the pid and status of child process" do
+      pidf = Process.fork { Process.exit! 99 }
+      results = Process.wait2
+      results.size.should == 2
+      pidw, status = results
+      pidf.should == pidw
+      status.exitstatus.should == 99
     end
   end
+
+  it "raises a StandardError if no child processes exist" do
+    lambda { Process.wait2 }.should raise_error(Errno::ECHILD)
+    lambda { Process.wait2 }.should raise_error(StandardError)
+  end
 end