xgettext: In language awk, avoid stack overflow.

xgettext: In language awk, avoid stack overflow.

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