tools/perf/build: Exclude MAKEFLAGS from nested invocation

In case the user specifies MAKEFLAGS as an environment variable,
or uses 'make -jN' explicitly, the options can conflict and result in:

    BUILD: Doing 'make -j8' parallel build
    make[1]: warning: -jN forced in submake: disabling jobserver mode.
    GEN common-cmds.h
    make[1]: *** write jobserver: Bad file descriptor.  Stop.

Make sure we invoke the main makefile in a pristine state.

Users who want to do something non-standard can use the:

  make -f Makefile.perf

method to invoke the makefile.

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-uen6hzTvkqqngqwjma9yoEgw@git.kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

tools/perf/Makefile

+#
+# This is a simple wrapper Makefile that calls the main Makefile.perf
+# with a -j option to do parallel builds
+#
+# If you want to invoke the perf build in some non-standard way then
+# you can use the 'make -f Makefile.perf' method to invoke it.
+#
 
 #
 # Clear out the built-in rules GNU make defines by default (such as .o targets),
@@ -5,6 +12,11 @@
 #
 .SUFFIXES:
 
+#
+# We don't want to pass along options like -j:
+#
+unexport MAKEFLAGS
+
 #
 # 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.
@@ -18,14 +30,12 @@ ifeq ($(JOBS),)
   endif
 endif
 
-export JOBS
-
 define print_msg
   @printf '    BUILD: Doing '\''make \033[33m-j'$(JOBS)'\033[m'\'' parallel build\n'
 endef
 
 define make
-  @$(MAKE) -f Makefile.perf --no-print-directory -j$(JOBS) $@
+  @$(MAKE) -f Makefile.perf --no-print-directory -j$(JOBS) O=$(O) $@
 endef
 
 #