test: conditionally test compact depending on configured support

I still have one machine with old enough Xapian to not have compaction
support. Make the tests check for unsupported compact operation when
compact is not available.

test/Makefile.local

@@ -35,9 +35,17 @@ $(dir)/symbol-test: $(dir)/symbol-test.o lib/$(LINKER_NAME)
 $(dir)/parse-time: $(dir)/parse-time.o parse-time-string/parse-time-string.o
 	$(call quiet,CC) $^ -o $@
 
+$(dir)/have-compact: Makefile.config
+ifeq ($(HAVE_XAPIAN_COMPACT),1)
+	ln -sf /bin/true $@
+else
+	ln -sf /bin/false $@
+endif
+
 .PHONY: test check
 
 TEST_BINARIES=$(dir)/arg-test \
+	      $(dir)/have-compact \
 	      $(dir)/hex-xcode \
 	      $(dir)/random-corpus \
 	      $(dir)/parse-time \

test/T020-compact.sh

@@ -10,6 +10,17 @@ notmuch tag +tag1 \*
 notmuch tag +tag2 subject:Two
 notmuch tag -tag1 +tag3 subject:Three
 
+if ! ${TEST_DIRECTORY}/have-compact; then
+    test_begin_subtest "Compact unsupported: error message"
+    output=$(notmuch compact --quiet 2>&1)
+    test_expect_equal "$output" "notmuch was compiled against a xapian version lacking compaction support.
+Compaction failed: Unsupported operation"
+
+    test_expect_code 1 "Compact unsupported: status code" "notmuch compact"
+
+    test_done
+fi
+
 test_expect_success "Running compact" "notmuch compact --backup=${TEST_DIRECTORY}/xapian.old"
 
 test_begin_subtest "Compact preserves database"