libpqxx: bump to 6.0.0

Other changes:
- Add a hash for the license file.

- Change the URL from pqxx.org to the official GitHub repository, as releases
  are no longer being posted to the old download mirror.

- Remove the patch, this has been fixed in the upstream code for some time.

- Remove AUTORECONF, as it is no longer necessary because the patch has been
  removed.

- Require gcc >= 4.7, as C++11 is now mandatory:
  http://pqxx.org/development/libpqxx/



Signed-off-by: Adam Duskett <Adamduskett@outlook.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>