I don't expect this to be fixed, but it might help others (including my future self).

This change to cmake/lz4.cmake is needed so that -DWITH_LZ4=system works
It is only required on the fb-mysql-8.0.28 branch

diff --git a/cmake/lz4.cmake b/cmake/lz4.cmake
index 24fce84fd3e..f7512ae088e 100644
--- a/cmake/lz4.cmake
+++ b/cmake/lz4.cmake
@@ -51,7 +51,7 @@ MACRO (FIND_SYSTEM_LZ4)
FIND_LIBRARY(LZ4_SYSTEM_LIBRARY
NAMES lz4)
IF (LZ4_INCLUDE_DIR AND LZ4_SYSTEM_LIBRARY)

• CHECK_LZ4_VERSION(${PATH_TO_LZ4})

• CHECK_LZ4_VERSION(${LZ4_INCLUDE_DIR})
  SET(SYSTEM_LZ4_FOUND 1)
  SET(LZ4_LIBRARY ${LZ4_SYSTEM_LIBRARY})
  ENDIF()