Makefile: Do not generate files if "configure" has not been run yet

Makefile: Do not generate files if "configure" has not been run yet

When doing a "make -j10" in the vanilla QEMU source tree (without
running "configure" first), the Makefile currently generates two
files already, qemu-version.h and qemu-options.def. This should not
happen, so let's only build the generated files if config-host.mak
is available (i.e. "configure" has been run already).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1496926799-13040-1-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>