crypto: require libgcrypt >= 1.5.0 for building QEMU

libgcrypt 1.5.0 was released in 2011 and all the distros that are build
target platforms for QEMU [1] include it:

  RHEL-7: 1.5.3
  Debian (Stretch): 1.7.6
  Debian (Jessie): 1.6.3
  OpenBSD (ports): 1.8.2
  FreeBSD (ports): 1.8.3
  OpenSUSE Leap 15: 1.8.2
  Ubuntu (Xenial): 1.6.5
  macOS (Homebrew): 1.8.3

Based on this, it is reasonable to require libgcrypt >= 1.5.0 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>