xgettext: In language C, avoid stack overflow.

xgettext: In language C, avoid stack overflow.

* gettext-tools/src/x-c.c (MAX_NESTING_DEPTH): New macro.
(nesting_depth): New variable.
(extract_parenthesized): Increase and check nesting_depth before calling
extract_parenthesized recursively.
(extract_whole_file): Initialize nesting_depth.
* gettext-tools/tests/xgettext-c-stackovfl-1: New file.
* gettext-tools/tests/xgettext-c-stackovfl-2: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add them.