perf tools: Fix 'make help' message error

Currently 'make help' message has such hint:

   use "make prefix=<path> <install target>" to install to a particular
       path like make prefix=/usr/local install install-doc

But this is misleading, when I specify "prefix=/usr/local", it has got no
respect at all.

This is because that, "DESTDIR" is considered first. In this case, "DESTDIR"
has an empty value, so "prefix" is honored. However, "prefix" is unconditionally
assigned to $HOME, regardless of what it is set to from command line. So our
"prefix" setting got no respect and the actual destination falls back to $HOME.

This patch fixes this issue and corrects the help message.

Signed-off-by: Jianyu Zhan <nasa4836@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1401727474-19370-1-git-send-email-nasa4836@gmail.com
Signed-off-by: Jiri Olsa <jolsa@kernel.org>

tools/perf/Makefile.perf

@@ -789,8 +789,8 @@ help:
 	@echo ''
 	@echo 'Perf install targets:'
 	@echo '  NOTE: documentation build requires asciidoc, xmlto packages to be installed'
-	@echo '  HINT: use "make prefix=<path> <install target>" to install to a particular'
-	@echo '        path like make prefix=/usr/local install install-doc'
+	@echo '  HINT: use "prefix" or "DESTDIR" to install to a particular'
+	@echo '        path like "make prefix=/usr/local install install-doc"'
 	@echo '  install	- install compiled binaries'
 	@echo '  install-doc	- install *all* documentation'
 	@echo '  install-man	- install manpage documentation'

tools/perf/config/Makefile

@@ -600,7 +600,7 @@ endif
 
 # Make the path relative to DESTDIR, not to prefix
 ifndef DESTDIR
-prefix = $(HOME)
+prefix ?= $(HOME)
 endif
 bindir_relative = bin
 bindir = $(prefix)/$(bindir_relative)