xgettext: In language EmacsLisp, avoid stack overflow, part 2.

xgettext: In language EmacsLisp, avoid stack overflow, part 2.

* gettext-tools/src/x-elisp.c (escape_nesting_depth): New variable.
(do_getc_escaped): Increase escape_nesting_depth before calling do_getc_escaped
recursively. Check escape_nesting_depth.
(extract_elisp): Initialize escape_nesting_depth.
* gettext-tools/tests/xgettext-elisp-stackovfl-3: New file.
* gettext-tools/tests/xgettext-elisp-stackovfl-4: New file.
* gettext-tools/tests/testdata/xg-el-so-3.el: New file.
* gettext-tools/tests/testdata/xg-el-so-4.el: New file.
* gettext-tools/tests/Makefile.am (TESTS, EXTRA_DIST): Add them.