/usr/include/synthesis/ is in libsynthesis-dev 3.4.0.16.6-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/synthesis/SDK_support.h
/usr/include/synthesis/SDK_util.h
/usr/include/synthesis/UI_util.h
/usr/include/synthesis/admindata.h
/usr/include/synthesis/blobs.h
/usr/include/synthesis/dataconversion.h
/usr/include/synthesis/dbitem.h
/usr/include/synthesis/engine_defs.h
/usr/include/synthesis/enginemodulebase.h
/usr/include/synthesis/enginemodulebridge.h
/usr/include/synthesis/generic_types.h
/usr/include/synthesis/lineartime.h
/usr/include/synthesis/prefix_file.h
/usr/include/synthesis/san.h
/usr/include/synthesis/stringutil.h
/usr/include/synthesis/syerror.h
/usr/include/synthesis/sync_dbapi.h
/usr/include/synthesis/sync_dbapidef.h
/usr/include/synthesis/sync_declarations.h
/usr/include/synthesis/sync_include.h
/usr/include/synthesis/sync_uiapi.h
/usr/include/synthesis/syncexception.h
/usr/include/synthesis/sysync_b64.h
/usr/include/synthesis/sysync_md5.h
/usr/include/synthesis/sysync_utils.h
/usr/include/synthesis/target_options.h
/usr/include/synthesis/timeutil.h