tools/perf/build: Automatically build in parallel, based on number of CPUs in the system

Implement automatic parallel builds when building in tools/perf:

  $ time make

  # [ perf build: Doing 'make -j12' parallel build. ]

  Auto-detecting system features:

  ...

  real    0m9.265s
  user    0m59.888s
  sys     0m6.082s

On GNU make achieving this is not particularly easy, it requires a separate
makefile, which then invokes the main Makefile.

( Note: this patch adds Makefile.parallel to show the concept - the two
  makefiles will be flipped in the next patch to avoid having to specify -f
  to get parallelism in the default build. )

Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/n/tip-dvBjwqiTyzrufzkz8oanhpf9@git.kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

tools/perf/Makefile.parallel

+#
+# Do a parallel build with multiple jobs, based on the number of CPUs online
+# in this system: 'make -j8' on a 8-CPU system, etc.
+#
+# (To override it, run 'make JOBS=1' and similar.)
+#
+ifeq ($(JOBS),)
+  JOBS := $(shell grep -c ^processor /proc/cpuinfo 2>/dev/null)
+  ifeq ($(JOBS),)
+    JOBS := 1
+  endif
+endif
+
+export JOBS
+
+$(info $(shell printf '# [ perf build: Doing '\''make \033[33m-j'$(JOBS)'\033[m'\'' parallel build. ]\n'))
+
+#
+# Needed if no target specified:
+#
+all:
+	@$(MAKE) --no-print-directory -j$(JOBS) $@
+
+%:
+	@$(MAKE) --no-print-directory -j$(JOBS) $@
+