Log Message:
zstd: updated to 1.3.7

Zstandard v1.3.7
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