Mine

jsled@phoenix [~/stuff/proj/gnucash/src-trunk]$ svn diff configure.in
Index: configure.in
===================================================================
--- configure.in	(revision 15400)
+++ configure.in	(working copy)
@@ -39,6 +39,14 @@
 
 ##  Do this first, because the other tests depend on it:
 
+USER_OPTIMIZATION=""
+for flag in $CFLAGS; do
+  if test -z $(echo $flag | sed -e 's,-O.,,'); then
+    USER_OPTIMIZATION="$USER_OPTIMIZATION ${flag}"
+  fi
+done
+echo "user_opt $USER_OPTIMIZATION"
+
 # Check for gcc and intltool (somehow breaks the suggested content
 # order, doesn't it? Whatever.)
 AC_PROG_CC
@@ -544,7 +552,10 @@
 
 AC_ARG_ENABLE( debug,
   [  --enable-debug               compile with debugging flags set],
-       CFLAGS="${CFLAGS} -g"
+       # remove any optimization flags
+       CFLAGS=$(echo $CFLAGS | sed -e 's,-O.,,g')
+       # ...except for those the user wants.
+       CFLAGS="${CFLAGS} -g ${USER_OPTIMIZATION}"
        LDFLAGS="${LDFLAGS} -g"
        AC_DEFINE(DEBUG_MEMORY,1,Enable debug memory),
        AC_DEFINE(DEBUG_MEMORY,0,Enable debug memory) )