xgettext: In language Perl, avoid stack overflow.

xgettext: In language Perl, avoid stack overflow.

* gettext-tools/src/x-perl.c (MAX_NESTING_DEPTH): New macro.
(nesting_depth): New variable.
(interpolate_keywords): Increase and check nesting_depth. Revert it before
returning.
(x_perl_lex): Likewise.
(extract_balanced): Increase and check nesting_depth before calling
extract_balanced recursively.
(extract_perl): Initialize nesting_depth.
* gettext-tools/tests/xgettext-perl-stackovfl-1: New file.
* gettext-tools/tests/xgettext-perl-stackovfl-2: New file.
* gettext-tools/tests/xgettext-perl-stackovfl-3: New file.
* gettext-tools/tests/xgettext-perl-stackovfl-4: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add them.