execute_cmd: always search for special builtins during simple command exec

This allows externally registered special builtins to always override
functions even when bash isn't built with strict posix mode enabled.

This will also have the affect that special builtins native to bash,
e.g. exit and source (see the full list via `enable -s`), won't be able
to be overridden by functions which in our case shouldn't matter and
probably is a good thing anyway.

Author: radhermit

execute_cmd.c

@@ -4670,7 +4670,7 @@ execute_simple_command (SIMPLE_COM *simple_command, int pipe_in, int pipe_out, i
 	 being used, and we don't want to exit the shell if a special
 	 builtin executed with `command builtin' fails.  `command' is not
 	 a special builtin. */
-      if (posixly_correct)
+      if (posixly_correct || BUILD_LIBRARY)
 	{
 	  builtin = find_special_builtin (words->word->word);
 	  if (builtin)