perf tools: Add EXCLUDE_EXTLIBS and EXTRA_PERFLIBS to makefile

perf tools: Add EXCLUDE_EXTLIBS and EXTRA_PERFLIBS to makefile

The goal is to allow users to override linking of libraries that
were automatically added to PERFLIBS.

EXCLUDE_EXTLIBS contains linker flags to be removed from LIBS
while EXTRA_PERFLIBS contains linker flags to be added.

My use case is to force certain library to be build statically,
e.g. for libelf:

  EXCLUDE_EXTLIBS=-lelf EXTRA_PERFLIBS=path/libelf.a

Signed-off-by: David Carrillo-Cisneros <davidcc@google.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Elena Reshetova <elena.reshetova@intel.com>
Cc: Kees Kook <keescook@chromium.org>
Cc: Paul Turner <pjt@google.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20170719011839.99399-3-davidcc@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>