Makefile: Deal with "tools: Refactor "xentoollog" into its own library"

Makefile: Deal with "tools: Refactor "xentoollog" into its own library"

In Xen 4.7 commit 5d3dc8671521ea4a4f753e77d3e7fb3a3a6f5f80
("tools: Refactor "xentoollog" into its own library") puts an
requirement on Python code to compile with:

-Wl,-rpath-link="+PATH_LIBXENTOOLLOG"

Which ends up with:

error: -Wl,-rpath-link=../../tools/libs/toollog: No such file or directory

We can't over-write the PATH_LIBXENTOOLLOG .. and on further
thought - there is absolutly no need for us to even build Python
code as we do not need Xend.

As such if we detect Xen 4.7 or later we skip this part.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>