Use Buildroot's setting for optimization, zstd's build system
overrides CFLAGS (adds -O3), but MOREFLAGS can override again.
Quick tests show that using -O2 (like buildroot)
is actually a little faster than -O3 on x86_64 Atoms.
Disable the legacy format, these are just needed for
decompressing files created with pre-release version.
Signed-off-by: Norbert Lange <nolange79@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
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