don't override user cflags

--- a/configure
+++ b/configure
@@ -2995,10 +2995,6 @@ fi
 # End of CC checks
 # After here, no more $cc or $ld runs
 
-if test "$debug" = "no" ; then
-  CFLAGS="-O2 -D_FORTIFY_SOURCE=2 $CFLAGS"
-fi
-
 # Disable zero malloc errors for official releases unless explicitly told to
 # enable/disable
 if test -z "$zero_malloc" ; then