configure: Omit fsanitize flags if they are already specified by the user

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

configure

@@ -3942,7 +3942,7 @@ set >> $logfile
 
 test -n "$valgrind" && toolchain="valgrind-memcheck"
 
-enabled ossfuzz && {
+enabled ossfuzz && ! echo $CFLAGS | grep -q -- "-fsanitize="  &&{
     add_cflags  -fsanitize=address,undefined -fsanitize-coverage=trace-pc-guard,trace-cmp -fno-omit-frame-pointer
     add_ldflags -fsanitize=address,undefined -fsanitize-coverage=trace-pc-guard,trace-cmp
 }