buildroot

package/libpqxx: update dependencies

As of v6.x, libpqxx takes advantage of thread support for transaction
sleeps (adding BR2_TOOLCHAIN_HAS_THREADS).

Re-order dependencies for:
 - Key feature (postgresql)
 - Architectures dependencies
 - Remaining dependencies

Fixes:

  http://autobuild.buildroot.net/results/e1d6f62edfc5eae90953f0a93fdb30d713409c37/

Signed-off-by: James Knight <james.knight@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>