Zstandard v1.3.7

This is minor fix release building upon v1.3.6.

The main reason we publish this new version is that @indygreg detected an important compression ratio regression for a specific scenario (compressing with dictionary at level 9 or 10 for small data, or 11 - 12 for large data) . We don't anticipate this scenario to be common : dictionary compression is still rare, then most users prefer fast modes (levels <=3), a few rare ones use strong modes (level 15-19), so "middle compression" is an extreme rarity.
But just in case some user do, we publish this release.

A few other minor things were ongoing and are therefore bundled.

Decompression speed might be slightly better with clang, depending on exact target and version. We could observe as mush as 7% speed gains in some cases, though in other cases, it's rather in the ~2% range.

The integrated backtrace functionality in the cli is updated : its presence can be more easily controlled, invoking BACKTRACE build macro. The automatic detector is more restrictive, and release mode builds without it by default. We want to be sure the default make compiles without any issue on most platforms.

Finally, the list of man pages has been completed with documentation for zstdless and zstdgrep, by @samrussell .

Detailed list of changes

• perf: slightly better decompression speed on clang (depending on hardware target)
• fix : ratio for dictionary compression at levels 9 and 10, reported by @indygreg
• build: no longer build backtrace by default in release mode; restrict further automatic mode
• build: control backtrace support through build macro BACKTRACE
• misc: added man pages for zstdless and zstdgrep, by @samrussell