tools lib api: Add debug output support

Adding support for warning/info/debug output within libapi code. Adding
following macros:

  pr_warning(fmt, ...)
  pr_info(fmt, ...)
  pr_debug(fmt, ...)

Also adding libapi_set_print function to set above functions. This will
be used in perf to set standard debug handlers for libapi.

Adding 2 header files:
  debug.h
    - to be used outside libapi, contains
      libapi_set_print interface

  debug-internal.h
    - to be used within libapi, contains
      pr_warning/pr_info/pr_debug definitions

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1455465826-8426-2-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/api/Makefile

@@ -18,6 +18,7 @@ LIBFILE = $(OUTPUT)libapi.a
 CFLAGS := $(EXTRA_WARNINGS) $(EXTRA_CFLAGS)
 CFLAGS += -ggdb3 -Wall -Wextra -std=gnu99 -Werror -O6 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -fPIC
 CFLAGS += -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64
+CFLAGS += -I$(srctree)/tools/lib/api
 
 RM = rm -f