If from a clean GIT checkout 'make -j 8' is run, the ESX
and Hyper-V code will be generated multiple times over.
This is because there are multiple files being generated
from one invocation of the generator script. make does not
realize this and so invokes the generator once per file.
This doesn't matter with serialized builds, but with
parallel builds multiple instances of the generator get
run at once.

make[2]: Entering directory `/home/berrange/src/virt/libvirt/src'
  GEN    util/virkeymaps.h
  GEN    remote/remote_protocol.h
  GEN    remote/remote_client_bodies.h
  GEN    remote/qemu_protocol.h
  GEN    remote/qemu_client_bodies.h
  GEN    esx/esx_vi_methods.generated.c
  GEN    esx/esx_vi_methods.generated.h
  GEN    esx/esx_vi_methods.generated.macro
  GEN    esx/esx_vi_types.generated.c
  GEN    esx/esx_vi_types.generated.h
  GEN    esx/esx_vi_types.generated.typedef
  GEN    esx/esx_vi_types.generated.typedef
  GEN    esx/esx_vi_types.generated.typeenum
  GEN    esx/esx_vi_types.generated.typetostring
  GEN    esx/esx_vi_types.generated.typefromstring
  GEN    esx/esx_vi_types.generated.h
  GEN    esx/esx_vi_types.generated.c
  GEN    esx/esx_vi_methods.generated.h
  GEN    esx/esx_vi_methods.generated.c
  GEN    esx/esx_vi_methods.generated.macro
  GEN    esx/esx_vi.generated.h
  GEN    esx/esx_vi.generated.c
  GEN    esx/esx_vi_types.generated.typeenum
  GEN    esx/esx_vi_types.generated.typedef
  GEN    esx/esx_vi_types.generated.typeenum
  GEN    esx/esx_vi_types.generated.typetostring
  GEN    esx/esx_vi_types.generated.typefromstring
  GEN    esx/esx_vi_types.generated.h
  GEN    esx/esx_vi_types.generated.c
  GEN    esx/esx_vi_methods.generated.h
  ...snip...
  GEN    hyperv/hyperv_wmi.generated.h
  GEN    libvirt_qemu_probes.h
  GEN    locking/qemu-sanlock.conf
  GEN    hyperv/hyperv_wmi.generated.c
  GEN    rpc/virnetprotocol.h
  GEN    hyperv/hyperv_wmi_classes.generated.typedef
  GEN    hyperv/hyperv_wmi_classes.generated.h
  GEN    hyperv/hyperv_wmi_classes.generated.c
  GEN    rpc/virkeepaliveprotocol.h
  GEN    remote/remote_protocol.c
  GEN    remote/qemu_protocol.c
  GEN    rpc/virkeepaliveprotocol.c
  GEN    rpc/virnetprotocol.c
  GEN    libvirt.def

Prevent this using a timestamp file to control generation,
as was previously done for the python bindings in commit
a7868e01Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>