Maintainer-mode stack traces: print symbolic names for errno.h errors, too.

* build/generator/gen_base.py
  (GeneratorBase.write_errno_table): New function.
  (IncludeDependencyInfo._scan_for_includes): Add exemption.

* gen-make.py
  (main): Invoke the new logic.

* subversion/libsvn_subr/errorcode.inc: New svn:ignore'd file.

* subversion/libsvn_subr/error.c
  (errorcode.inc): Include, in maintainer mode.
  (svn__errno): New file-private variable, defined by errorcode.inc.
  (svn_error_symbolic_name): Try svn__errno too.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1467643 13f79535-47bb-0310-9956-ffa450edef68