Let this compile and run (with reduced capability) on R3.13.

msi.c

@@ -26,6 +26,10 @@
 
 #define MAX_BUFFER_SIZE 4096
 
+#if ((EPICS_VERSION <= 3) && (EPICS_REVISION <= 13))
+#define macEnvExpand(x) strdup(x)
+#endif
+
 /*Forward references to local routines*/
 static void usageExit(void);
 static void addMacroReplacements(MAC_HANDLE *macPvt,char *pval);

msi.html

@@ -161,8 +161,9 @@ it specified it is used, otherwise the file template is used. This format is
 an extension of the format accepted by dbLoadTemplate. It allows templates to
 be  expanded on the host rather via dbLoadTemplate. The file name may
 appear inside double quotation marks; these are only required if the name
-contains any environment variable macros of the form ${MOTOR} which will be
-expanded before the file is opened. 
+contains any environment variable macros of the form ${ENV_VAR} which will be
+expanded before the file is opened. Environment variable macro expansion
+is available when msi is linked with EPICS base R3.14.3 or newer.
 
 <h3>Regular substitution example</h3>
 Let the command be: