gd: update to version 2.1.0

Changes:
- Add DESCRIPTION and HOMEPAGE
- libgd.org is down, use bitbucket.org instead and also
  remove the MIRROR for it.
- Remove the unnecessary specified dir for --with-png.

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>