move Environment#shell to BasicEnvironment; clean up a few yard warnings

lib/rscons/basic_environment.rb

@@ -247,5 +247,28 @@ module Rscons
       end
     end
 
+    # Execute a command using the system shell.
+    #
+    # The shell is automatically determined but can be overridden by the SHELL
+    # construction variable. If the SHELL construction variable is specified,
+    # the flag to pass to the shell is automatically dtermined but can be
+    # overridden by the SHELLFLAG construction variable.
+    #
+    # @param command [String] Command to execute.
+    #
+    # @return [String] The command's standard output.
+    def shell(command)
+      shell_cmd =
+        if shell = get_var("SHELL")
+          flag = get_var("SHELLFLAG") || (shell == "cmd" ? "/c" : "-c")
+          [shell, flag]
+        else
+          Rscons.get_system_shell
+        end
+      IO.popen([*shell_cmd, command]) do |io|
+        io.read
+      end
+    end
+
   end
 end