Relax the static assertion regarding
`ZstdWriterBase::Options::kMinTargetCBlockSize` and
`ZSTD_TARGETCBLOCKSIZE_MIN`.

This allows building with an older zstd where `ZSTD_TARGETCBLOCKSIZE_MIN`
was 64.

PiperOrigin-RevId: 936069594

riegeli/zstd/zstd_writer.cc

diff --git a/riegeli/zstd/zstd_writer.cc b/riegeli/zstd/zstd_writer.cc
index 3813554..6466116 100644
--- a/riegeli/zstd/zstd_writer.cc
+++ b/riegeli/zstd/zstd_writer.cc
@@ -47,7 +47,8 @@
 
 static_assert(ZstdWriterBase::Options::kMinWindowLog == ZSTD_WINDOWLOG_MIN);
 static_assert(ZstdWriterBase::Options::kMaxWindowLog == ZSTD_WINDOWLOG_MAX);
-static_assert(ZstdWriterBase::Options::kMinTargetCBlockSize ==
+// `ZSTD_TARGETCBLOCKSIZE_MIN` was 64 before zstd-1.5.6.
+static_assert(ZstdWriterBase::Options::kMinTargetCBlockSize >=
               ZSTD_TARGETCBLOCKSIZE_MIN);
 static_assert(ZstdWriterBase::Options::kMaxTargetCBlockSize ==
               ZSTD_TARGETCBLOCKSIZE_MAX);