Backporting missing pieces of plugin building infrastructure:

  - Adding Makefile 'plugins' target to build all
    defined plugins

  - Adding Makefile 'install_plugins' target as 'install_lib'
    target dependency

  - Link plugin objects with shared object building

Backported from Steven Rostedt's trace-cmd repo (HEAD 0f2c2fb):
git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/trace-cmd.git

Plugins are by default installed into following locations:

  '$(HOME)/.traceevent/plugins'
     - If we are installing under $(HOME)

  '$(prefix)/lib/traceevent/plugins'
     - Otherwise

This path is propagated to the plugin object as a plugins search path.
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1386076182-14484-4-git-send-email-jolsa@redhat.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>