build: don't use recursive make to build the 'doc' subdirectory

build: don't use recursive make to build the 'doc' subdirectory

* doc/Makefile.am: Rename ...
* doc/local.mk: ... like this.  With further adjustments ...
(info_TEXINFOS): Prepend 'doc/' to all '*.texi' files listed in
here.
(coreutils_TEXINFOS): Likewise, and rename ...
(doc_coreutils_TEXINFOS): ... like this.
(constants.texi): Rename ...
(doc/constants.texi): ... like this.  Adjust the recipe to avoid
spurious errors.
(MAINTAINERCLEANFILES): Adjust, and extend with '+=' rather than
setting it with '='.
(ME): Delete.
(find_upper_case_var): Use '$@', not '$(ME)', in error messages.
* Makefile.am: Include 'doc/local.mk'.
(SUBDIRS): Remove 'doc'.
* configure.ac (AC_CONFIG_FILES): Remove 'doc/Makefile'.