[doit][darwin] make sure compiles are run with -O2

Make sure the default -g -O2 flags are passed on darwin when specifying the
-fbracket-depth switch, which is still needed for ARM compiles.

Change-Id: I35193131ddabbb4964e4b6beba432ab88a226432
diff --git a/doit b/doit
index 4780121..8c8fe61 100755
--- a/doit
+++ b/doit
@@ -93,7 +93,7 @@
 fi
 if [ "$OS" = "Darwin" ]; then
     # needed to work around problem with clang compiler and some generated code in gcc
-    export CXXFLAGS="-fbracket-depth=1024"
+    export CXXFLAGS="-fbracket-depth=1024 -g -O2"
 fi
 MAKE=make
 if [ "$OS" = "FreeBSD" ]; then