Update curl library to curl-7.60.0 (#20181)

* Update curl library to curl-7.60.0

This fix updates curl library to 7.60.0.
(Previously TensorFlow links to curl 7.49.1, which was
relesed in 2016)

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Update source files in curl

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add missing flag for curl 7.60.0

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add missing include "system.h"

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>