Fix gntshr build against vanilla xen 4.1: stubs will throw an exception at run-time

Note that if we backport the gntshr xen patches to 4.1, we should also add
  #define HAVE_GNTSHR 1
to xenctrl.h.

In the long term should we use autoconf to detect the presence of the
actual functions we need?

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Remove redundancy in Makefile

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Improve .gitignore
Merge pull request #2 from djs55/gntshr-41

Fix the build on xen 4.1 when gntshr is missing