Revert "fix nettle"
This reverts commit d31634c3. Laurent Thomas had a problem on one machine with the build_oai way of checking for nettle. The problem with the alternative solution of including nettle/bignum.h is that it is very unclear. The problem with nettle is that the file nettle/config.h does not exist for version 2. It was introduced in version 3. We want to support both versions, but there is an API incompatibility. So we need an #if #else mechanism. The file nettle/bignum.h is present in both versions 2 and 3 and it includes nettle/version.h in the version 3. So by including this file, we can check for the existence of NETTLE_VERSION_MAJOR (that comes from nettle/config.h) in the code. But as you can see, the reasoning is way too complex. So it's better to keep the check in cmake_targets/CMakeLists.txt. As long as we support version 2 this will be the way to go. It is possible to force a given version in specific non-generic customized environments.