tools/perf/build: Speed up the final link

libtraceevent.a and liblk.a rules have always-missed dependencies,
which causes python.so to be relinked at every build attempt - even
if none of the affected code changes.

This slows down re-builds unnecessarily, by adding more than a second
to the build time:

  comet:~/tip/tools/perf> time make

  ...

    SUBDIR /fast/mingo/tip/tools/lib/lk/
    make[1]: `liblk.a' is up to date.
    SUBDIR /fast/mingo/tip/tools/lib/traceevent/
    LINK perf
    GEN python/perf.so

  real    0m1.701s
  user    0m1.338s
  sys     0m0.301s

Add the (trivial) dependencies to not force a re-link.

This speeds up an empty re-build enormously:

  comet:~/tip/tools/perf> time make

  ...

  real    0m0.207s
  user    0m0.134s
  sys     0m0.028s

[ This adds some coupling between the build dependencies of
  libtraceevent and liblk - but until those stay relatively
  simple this should not be an issue. ]

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-wvmlrurufuk6mo1ovtNigguT@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/perf/Makefile

@@ -669,15 +669,19 @@ $(LIB_FILE): $(LIB_OBJS)
 	$(QUIET_AR)$(RM) $@ && $(AR) rcs $@ $(LIB_OBJS)
 
 # libtraceevent.a
-$(LIBTRACEEVENT):
+TE_SOURCES = $(wildcard $(TRACE_EVENT_DIR)*.[ch])
+
+$(LIBTRACEEVENT): $(TE_SOURCES)
 	$(QUIET_SUBDIR0)$(TRACE_EVENT_DIR) $(QUIET_SUBDIR1) O=$(OUTPUT) libtraceevent.a
 
 $(LIBTRACEEVENT)-clean:
 	$(QUIET_SUBDIR0)$(TRACE_EVENT_DIR) $(QUIET_SUBDIR1) O=$(OUTPUT) clean
 
+LIBLK_SOURCES = $(wildcard $(LK_PATH)*.[ch])
+
 # if subdir is set, we've been called from above so target has been built
 # already
-$(LIBLK):
+$(LIBLK): $(LIBLK_SOURCES)
 ifeq ($(subdir),)
 	$(QUIET_SUBDIR0)$(LK_DIR) $(QUIET_SUBDIR1) O=$(OUTPUT) liblk.a
 endif
@@ -825,6 +829,6 @@ else
     GIT-HEAD-PHONY =
 endif
 
-.PHONY: all install clean strip $(LIBTRACEEVENT) $(LIBLK)
+.PHONY: all install clean strip
 .PHONY: shell_compatibility_test please_set_SHELL_PATH_to_a_more_modern_shell
 .PHONY: $(GIT-HEAD-PHONY) TAGS tags cscope .FORCE-PERF-CFLAGS