PR 117812 reports that testing GCC with zstd 1.3.4 fails because
ZSTD_CLEVEL_DEFAULT is not defined, so avoid using it.

	PR libbacktrace/117812
	* zstdtest.c (test_large): Use 3 rather than ZSTD_CLEVEL_DEFAULT

zstdtest has some inline data where some testcases lack the
uncompressed length field.  Thus it computes that but still
ends up allocating memory for the uncompressed buffer based on
that (zero) length.  Oops.  Causes memory corruption if the
allocator returns non-NULL.

libbacktrace/
	* zstdtest.c (test_samples): Properly compute the allocation
	size for the uncompressed data.

	Support decompressing --compress-debug-sections=zstd.
	* configure.ac: Check for zstd library and
	--compress-debug-sections=zstd linker option.
	* Makefile.am (zstdtest_*): New targets.
	(zstdtest_alloc_*, ctestzstd_*): New targets.
	(BUILDTESTS): Add zstdtest, zstdtest_alloc, ctestzstd as
	appropriate.
	* elf.c (ELFCOMPRESS_ZSTD): Define.
	(elf_fetch_bits): Rename from elf_zlib_fetch.  Update uses.
	(elf_fetch_bits_backward): New static function.
	(ZLIB_HUFFMAN_*): Rename from HUFFMAN_*.  Update uses.
	(ZLIB_TABLE_*): Rename from ZDEBUG_TABLE_*.  Update uses.
	(ZSTD_TABLE_*): Define.
	(struct elf_zstd_fse_entry): Define.
	(elf_zstd_read_fse): New static function.
	(elf_zstd_build_fse): Likewise.
	(lit): Define if BACKTRACE_GENERATE_ZSTD_FSE_TABLES.
	(match, offset, next, print_table, main): Likewise.
	(elf_zstd_lit_table): New static const array.
	(elf_zstd_match_table, elf_zstd_offset_table): Likewise.
	(elf_zstd_read_huff): New static function.
	(struct elf_zstd_seq_decode): Define.
	(elf_zstd_unpack_seq_decode): New static function.
	(ZSTD_LIT_*): Define.
	(struct elf_zstd_literals): Define.
	(elf_zstd_literal_output): New static function.
	(ZSTD_LITERAL_LENGTH_BASELINE_OFFSET): Define.
	(elf_zstd_literal_length_baseline): New static const array.
	(elf_zstd_literal_length_bits): Likewise.
	(ZSTD_MATCH_LENGTH_BASELINE_OFFSET): Define.
	(elf_zstd_match_length_baseline): New static const array.
	(elf_zstd_match_length_bits): Likewise.
	(elf_zstd_decompress): New static function.
	(ZDEBUG_TABLE_SIZE): New definition.
	(elf_uncompress_chdr): Support ELF_COMPRESS_ZSTD.
	(backtrace_uncompress_zstd): New function.
	(elf_add): Use ZLIB_TABLE_SIZE for zlib-gnu sections.
	* internal.h (backtrace_uncompress_zstd): Declare.
	* zstdtest.c: New file.
	* configure, config.h.in, Makefile.in: Regenerate.