Modernize: Use gnulib's string-buffer module for accumulating strings.

Modernize: Use gnulib's string-buffer module for accumulating strings.

* autogen.sh (GNULIB_MODULES_RUNTIME_FOR_SRC, GNULIB_MODULES_TOOLS_FOR_SRC): Add
xstring-buffer.
* gettext-runtime/src/envsubst.c: Include string-buffer.h.
(subst_from_stdin): Use a stack-allocated string_buffer instead of 3 local
variables and micro-optimized ad-hoc code.
* gettext-tools/src/cldr-plural.y: Include string-buffer.h.
(yylex): Use a stack-allocated string_buffer instead of 3 local variables and
micro-optimized ad-hoc code.
* gettext-tools/src/cldr-plurals.c: Include string-buffer.h instead of xalloc.h.
(extract_rules): Use a stack-allocated string_buffer instead of 3 local
variables and micro-optimized ad-hoc code.
* gettext-tools/src/its.c: Include string-buffer.h.
(_its_collect_text_content): Use a stack-allocated string_buffer instead of 3
local variables and micro-optimized ad-hoc code.
* gettext-tools/src/read-desktop.c: Include string-buffer.h.
(desktop_lex): Use a stack-allocated string_buffer instead of 3 local variables
and micro-optimized ad-hoc code.
* gettext-tools/src/read-properties.c: Include string-buffer.h.
(properties_parse): Use a stack-allocated string_buffer instead of 3 local
variables and micro-optimized ad-hoc code.
* gettext-tools/src/x-awk.c: Include string-buffer.h.
(phase2_getc, phase3_get): Use a stack-allocated string_buffer instead of 3
local variables and micro-optimized ad-hoc code.
* gettext-tools/src/x-c.c: Include string-buffer.h.
(phase5_get): Use a stack-allocated string_buffer instead of 3 local variables
and micro-optimized ad-hoc code.
* gettext-tools/src/x-java.c: Include string-buffer.h.
(phase5_get): Use a stack-allocated string_buffer instead of 3 local variables
and micro-optimized ad-hoc code.
* gettext-tools/src/x-javascript.c: Include string-buffer.h.
(phase5_get): Use a stack-allocated string_buffer instead of 3 local variables
and micro-optimized ad-hoc code.
* gettext-tools/src/x-lua.c: Include string-buffer.h.
(string_buf_length, string_buf_alloc, string_buf): Remove variables.
(string_start, string_add, string_end): Remove functions.
(phase3_get): Use a stack-allocated string_buffer instead of 3 local variables
and micro-optimized ad-hoc code.
* gettext-tools/src/x-perl.c: Include string-buffer.h.
(get_here_document, phase2_getc, extract_quotelike_pass1,
extract_quotelike_pass3, extract_variable, interpolate_keywords, x_perl_prelex):
Use a stack-allocated string_buffer instead of 3 local variables and
micro-optimized ad-hoc code.
* gettext-tools/src/x-php.c: Include string-buffer.h.
(process_heredoc, phase4_get): Use a stack-allocated string_buffer instead of 3
local variables and micro-optimized ad-hoc code.
* gettext-tools/src/x-python.c: Include string-buffer.h.
(phase5_get): Use a stack-allocated string_buffer instead of 3 local variables
and micro-optimized ad-hoc code.
* gettext-tools/src/x-rst.c: Include string-buffer.h.
(buffer, bufmax): Remove variables.
(buffer): New variable.
(extract_rst, parse_integer): Use a stack-allocated string_buffer instead of 3
local variables and micro-optimized ad-hoc code.
(extract_rsj): Update.
* gettext-tools/src/x-smalltalk.c: Include string-buffer.h.
(phase2_get): Use a stack-allocated string_buffer instead of 3 local variables
and micro-optimized ad-hoc code.
* gettext-tools/src/x-vala.c: Include string-buffer.h.
(phase3_get): Use a stack-allocated string_buffer instead of 3 local variables
and micro-optimized ad-hoc code.
* gettext-tools/src/x-ycp.c: Include string-buffer.h.
(phase2_getc, phase5_get): Use a stack-allocated string_buffer instead of 3
local variables and micro-optimized ad-hoc code.