perf: Make the install relative to DESTDIR if specified

perf: Make the install relative to DESTDIR if specified

Without this change, the install path is relative to
prefix/DESTDIR where prefix is automatically set to $HOME.

This can produce unexpected results. For example:

  make -C tools/perf DESTDIR=/home/jkacur/tmp install-man

creates the directory:		/home/jkacur/home/jkacur/tmp/share/...
instead of the expected:	/home/jkacur/tmp/share/...

Signed-off-by: John Kacur <jkacur@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
Cc: Kyle McMartin <kyle@redhat.com>
Cc: <stable@kernel.org>
LKML-Reference: <1268312220-12880-1-git-send-email-jkacur@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>