Merge topic 'png-optimization-flags-include'

f4fdf16f Merge branch 'upstream-png' into png-optimization-flags-include
bf3ae807 png 2019-05-30 (f4232b5e)
e9087620 png: move the optimization disabling flags to the right place

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: Brad King <brad.king@kitware.com>
Merge-request: !5603