Index: /issm/trunk-jpl/m4/issm_options.m4
===================================================================
--- /issm/trunk-jpl/m4/issm_options.m4	(revision 25645)
+++ /issm/trunk-jpl/m4/issm_options.m4	(revision 25646)
@@ -195,7 +195,9 @@
 		*darwin*)
 			IS_MAC=yes
+			AC_DEFINE([_MAC_], [1], [is macOS])
 		;;
 		*)
 			IS_MAC=no
+			AC_DEFINE([_MAC_], [0], [is macOS])
 		;;
 	esac
@@ -204,11 +206,14 @@
 
 	if test "${IS_MAC}" == "yes"; then
-		AC_MSG_CHECKING([for fmemopen])
-		FMEMOPEN=no
+		AC_MSG_CHECKING([if system lib.c has fmemopen])
 		if [[ $(system_profiler SPSoftwareDataType | $(which grep) "System Version:" | sed -e "s/[[:space:]]*System Version: macOS //" | cut -d "." -f 2) -ge 13 ]]; then
-			FMEMOPEN=yes
-		fi
-		AM_CONDITIONAL([HAS_FMEMOPEN], [test "${FMEMOPEN}" == "yes"])
-		AC_MSG_RESULT([${FMEMOPEN}])
+			SYSTEM_FMEMOPEN="yes"
+			AC_DEFINE([_SYSTEM_FMEMOPEN_], [1], [does system lib.c have fmemopen])
+		else
+			SYSTEM_FMEMOPEN="no"
+			AC_DEFINE([_SYSTEM_FMEMOPEN_], [0], [does system lib.c have fmemopen])
+		fi
+		AM_CONDITIONAL([SYSTEM_HAS_FMEMOPEN], [test "${SYSTEM_FMEMOPEN}" == "yes"])
+		AC_MSG_RESULT([${SYSTEM_FMEMOPEN}])
 	fi