Don't override our compiler defaults.

https://bugs.gentoo.org/893592
--- a/inc/MyBuilder.pm
+++ b/inc/MyBuilder.pm
@@ -208,7 +208,6 @@ sub ACTION_create_objects {
         $object =~ s/\.c/.o/;
         next if $self->up_to_date($file, $object);
         $cbuilder->compile(object_file  => $object,
-                           extra_compiler_flags=>["-D_FORTIFY_SOURCE=1",@EXTRA_FLAGS],
                            source       => $file,
                            include_dirs => ["btparse/src"]);
     }