Makefile: fix -j race to create ./bup
Makefile: fix -j race to create ./bup

Previously the bup target might fail if it was run in parallel
(i.e. during make -j).  Since bup depends on _version.py, which is
phony, that wasn't unlikely.

Fix the race by ignoring errors while creating the symlink, and then
testing for existence afterward.

Signed-off-by: Rob Browning <1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org>
Tested-by: Rob Browning <1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org>

mhoeher/bup,pombredanne/bup,tjanez/bup,jbaber/bup,tjanez/bup,ToxicFrog/bup,gevaerts/bup,gevaerts/bup,mhoeher/bup,jbaber/bup,pombredanne/bup,tjanez/bup,gevaerts/bup,ToxicFrog/bup,mhoeher/bup,jbaber/bup,tjanez/bup,mhoeher/bup,ToxicFrog/bup,pombredanne/bup,mhoeher/bup,mhoeher/bup,gevaerts/bup,ToxicFrog/bup,tjanez/bup,ToxicFrog/bup,jbaber/bup,pombredanne/bup,gevaerts/bup,pombredanne/bup,jbaber/bup,tjanez/bup,jbaber/bup,pombredanne/bup