[3.6] make tags: index also Modules/_ctypes/ (#4648) (#4659)

[3.6] make tags: index also Modules/_ctypes/ (#4648) (#4659)

* `make tags` fixes (GH-717)

* Fix `make tags` warnings

`make tags` target tries to find C sources and headers in "Grammar" and
"Mac" folders and generates these warnings:

    ctags: Warning: cannot open source file "Grammar/*.[ch]" : No such file or directory
    ctags: Warning: cannot open source file "Mac/*.[ch]" : No such file or directory

This commit changes $SRCDIRS variable in configure.ac to remote these
directories. This variable is used only for tags generation.

Also, "configure" was regenerated with `autoreconf`.

* Fix `make tags` fail on non-default tag names

When ctags overrides default tags filename (e.g. `-f .tags`) `make tags`
is failed because it assumes to see default `tags` filename:

    sort: cannot read: tags: No such file or directory

This commit explicitly specifies "tags" filename for tags generation.

(cherry picked from commit 8a543c0bc7347d5b333f334d157bf4a7cd33c14a)

* make tags: index also Modules/_ctypes/ (#4648)

Avoid also "cd $(srcdir)" to not change the current directory.

(cherry picked from commit 3be3b97a9709d3cd5303175ddbffa7dcca57ac3e)