Fix bugs in my change to improve the handling of linking

Estimated hours taken: 0.1
Branches: main

Fix bugs in my change to improve the handling of linking
with a non-installed standard library.

scripts/mmake.in:
	Export MERCURY_STDLIB_DIR.

scripts/Mmake.rules:
	Always set MERC_INT_DIR to avoid undefined variable warnings.

scripts/Mmake.rules

@@ -51,6 +51,8 @@ else
     ifdef MERCURY_STDLIB_DIR
         export MERCURY_STDLIB_DIR
         MERC_INT_DIR = $(MERCURY_STDLIB_DIR)/ints
+    else
+        MERC_INT_DIR =
     endif
 endif
 

scripts/mmake.in

@@ -181,6 +181,7 @@ done
 if [ "$MERCURY_STDLIB_DIR" != "" ]
 then
 	MMAKE_DIR=${MMAKE_DIR=$MERCURY_STDLIB_DIR/mmake}
+	export MERCURY_STDLIB_DIR
 fi
 
 case "$MMAKE_DIR" in