xgettext: In language Scheme, avoid stack overflow.
* gettext-tools/src/x-scheme.c: Include error-progname.h.
(MAX_NESTING_DEPTH): New macro.
(nesting_depth): New variable.
(read_object): Increase nesting_depth before calling read_object recursively.
Check nesting_depth.
(extract_scheme): Initialize nesting_depth.
* gettext-tools/tests/xgettext-scheme-stackovfl-1: New file.
* gettext-tools/tests/xgettext-scheme-stackovfl-2: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add them.
