update for zstd support

lib/compress/Makefile.lammps

@@ -1,21 +1,24 @@
 # This file contains the settings to build and link LAMMPS with
 # support for data compression libraries.
-# 
+#
 # When you build LAMMPS with the COMPRESS package installed, it will
 # use the 3 settings in this file.  They should be set as follows.
-# 
-# The compress_SYSLIB setting is for linking the compression library.
-# By default, the setting will point to zlib (-lz).
-# 
-# The compress_SYSINC and compress_SYSPATH variables do not typically need
-# to be set, as compression libraries are usually installed as packages
-# in system locations. Otherwise, specify its directory via the
-# compress_SYSPATH variable, e.g. -Ldir or compress_SYSINC variable( -Idir)
+#
+# The compress_SYSLIB setting is for linking the compression libraries.
+# By default, the setting will point to zlib (-lz). For including
+# Zstandard support add -DLAMMPS_ZSTD to compress_SYSINC and also
+# add -lzstd to compress_SYSLIB to link to the library.
+#
+# The compress_SYSINC and compress_SYSPATH variables typically do not
+# need any additional settings, as compression libraries are usually
+# installed as packages in system locations. Otherwise, specify its
+# library directory via the compress_SYSPATH variable, e.g. -Ldir or
+# its include directory via the compress_SYSINC variable( -Idir)
 
 # -----------------------------------------------------------
 
 # Settings that the LAMMPS build will import when this package is installed
 
-compress_SYSINC =
-compress_SYSLIB = -lz
+compress_SYSINC = # -DLAMMPS_ZSTD
+compress_SYSLIB = -lz # -lzstd
 compress_SYSPATH =