xgettext: awk: Recognize string concatenation.

xgettext: awk: Recognize string concatenation.

* gettext-tools/src/x-awk.c (SIZEOF): New macro.
(phase3_pushback, phase3_pushback_length): New variables.
(phase3_get): Renamed from x_awk_lex. Return pushed-back token if present.
(phase3_unget): New function.
(string_concat_free1): New function.
(phase4_get): New function.
(extract_parenthesized): Invoke phase4_get instead of x_awk_lex.
(extract_awk): Initialize phase3_pushback_length.
* gettext-tools/tests/xgettext-awk-1: Add test cases for string concatenation.
* NEWS: Mention the improvement.