Enable '-Werror' on the tests.

test/Makefile.am

@@ -15,6 +15,7 @@ simple-db.db: simple-db.sql
 	$(DBGEN_V)$(SQLITE3_CMD) $@ < $(srcdir)/simple-db.sql
 
 AM_CPPFLAGS = \
+	-Wall \
 	-Werror=sign-compare \
 	-I$(top_srcdir)/src \
 	$(READLINE_CFLAGS) \

test/drive_sequencer.cc

@@ -87,7 +87,7 @@ private:
 
 int main(int argc, char *argv[])
 {
-    int c, retval = EXIT_SUCCESS;
+    int retval = EXIT_SUCCESS;
     const char *errptr;
     auto_fd fd;
     pcre *code;