configure: since the *BSDs have GNU make as gmake, look for that too.

Thanks to Ryan Hinton <iobass@email.com> for the report.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Move non-system includes after system includes in _helpers.c.

Having Python.h before assert.h was apparently causing trouble on
OpenBSD:

  _helpers.c: In function 'bup_xutime_ns':
  _helpers.c:842: warning: implicit declaration of function 'utimensat'
  _helpers.c:842: error: 'AT_FDCWD' undeclared (first use in this function)
  _helpers.c:842: error: (Each undeclared identifier is reported only once
  _helpers.c:842: error: for each function it appears in.)
  _helpers.c:843: error: 'AT_SYMLINK_NOFOLLOW' undeclared (first use in this function)
  error: command 'cc' failed with exit status 1

...and moving the non-sytem includes lower is reasonable anyway.

Thanks to Ryan Hinton <iobass@email.com> for the report.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Move _helper.c's Python.h back up (now to the top), because Python.

Move the Python.h include in _helper.c to the top because Python says
so:

  http://docs.python.org/2/c-api/intro.html#include-files

Though according to this, it's been fixed upstream:

  http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=206805

Which might explain why I didn't see the problem here.

Thanks to Chandan Kumar <chandank.kumar@gmail.com> for the report.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>