nettle 2.7.1 was released in 2013 and all the distros that are build
target platforms for QEMU [1] include it:
RHEL-7: 2.7.1
Debian (Stretch): 3.3
Debian (Jessie): 2.7.1
OpenBSD (ports): 3.4
FreeBSD (ports): 3.4
OpenSUSE Leap 15: 3.4
Ubuntu (Xenial): 3.2
macOS (Homebrew): 3.4
Based on this, it is reasonable to require nettle >= 2.7.1 in QEMU
which allows for some conditional version checks in the code to be
removed.
[1] https://qemu.weilnetz.de/doc/qemu-doc.html#Supported-build-platforms
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>