perf tools: Allow to specify lib compile variable for spec usage

We need a way to specify $(lib) part of the installation path for
traceevent plugin libraries. Currently we use 'lib64' for x86_64 and
'lib' otherwise.

Instead of listing all possible values, this change allows the rpm spec
code to specify the correct $(lib) part based on processed architecture,
like

  $ make ... lib=%{_lib}
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Kyle McMartin <kyle@mcmartin.ca>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Kyle McMartin <kyle@mcmartin.ca>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1408978552-17131-1-git-send-email-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/config/Makefile

@@ -653,11 +653,13 @@ else
 sysconfdir = $(prefix)/etc
 ETC_PERFCONFIG = etc/perfconfig
 endif
+ifndef lib
 ifeq ($(IS_X86_64),1)
 lib = lib64
 else
 lib = lib
 endif
+endif # lib
 libdir = $(prefix)/$(lib)
 
 # Shell quote (do not use $(call) to accommodate ancient setups);