build: respect CFLAGS when building warn_collector.l

We update common.mk to the latest (not really needed, by might as well).

Then we fix the lex building so it uses CFLAGS.  This in turn triggers a
clang warning in the file which we fix by adding proper printf attributes.

BUG=chromium:297661
TEST=`VERBOSE=1 emerge-x86-alex crash-reporter` showed flags used w/lex

Change-Id: I8db4b8b54b2ca748bb400ec6541287fa764f6d43
Reviewed-on: https://chromium-review.googlesource.com/170351
Reviewed-by: Yunlian Jiang <yunlian@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/crash_reporter/warn_collector.l b/crash_reporter/warn_collector.l
index 6fc2f56..5cb12f1 100644
--- a/crash_reporter/warn_collector.l
+++ b/crash_reporter/warn_collector.l
@@ -82,6 +82,7 @@
 const char *warn_dump_path = "/var/run/kwarn/warning";
 const char *crash_reporter_command;
 
+__attribute__((__format__(__printf__, 1, 2)))
 static void Die(const char *format, ...) {
   va_list ap;
   va_start(ap, format);