;tools: just functest: fail if there are warnings

(Does not force recompilation of already compiled modules, but should
help catch more warnings before pushing them to CI.)

Justfile

@@ -495,10 +495,10 @@ SHELLTEST := 'COLUMNS=80 ' + STACK + ' exec -- shelltest --execdir --exclude=/_
 
 #  --hide-successes
 
-# build hledger quickly and run functional tests, with any shelltest OPTS (requires mktestaddons)
-@functest *OPTS:
-    $STACK build hledger
-    time (({{ SHELLTEST }} {{ if OPTS == '' { '' } else { OPTS } }} \
+# build hledger warning-free and run functional tests, with any shelltest OPTS (requires mktestaddons)
+@functest *STOPTS:
+    $STACK build --ghc-options=-Werror hledger
+    time (({{ SHELLTEST }} {{ if STOPTS == '' { '' } else { STOPTS } }} \
         hledger/test/ bin/ \
         -x ledger-compat/ledger-baseline -x ledger-compat/ledger-regress -x ledger-compat/ledger-extra \
         && echo $@ PASSED) || (echo $@ FAILED; false))