perf_counter: copy in Git's top Makefile

We'd like to have a similar user-space structure as Git has, for the
perfcounter tools - so copy in Git's toplevel makefile as-is.

We'll strip it down in subsequent commits to make it fit the
perfcounters code.

The Git version used: 66996ec: Sync with 1.6.2.4
Signed-off-by: NIngo Molnar <mingo@elte.hu>