Run clone/fork/vfork tests with and without semaphores.

Run clone/fork/vfork tests with and without semaphores.

* testsuite/systemtap.clone/dtrace_clone.exp: Run the test twice - once
  without semaphores, once with semaphores.
* testsuite/systemtap.clone/dtrace_fork_exec.exp: Ditto.
* testsuite/systemtap.clone/dtrace_vfork_exec.exp: Ditto.
* testsuite/systemtap.clone/test_progs.tcl: Automatically run "make clean"
  when needed.  Allow callers to specify extra CFLAGS values.
* testsuite/systemtap.clone/dtrace_child.c: Allow semaphores to be
  ifdef'ed out.
* testsuite/systemtap.clone/dtrace_clone.c: Ditto.
* testsuite/systemtap.clone/dtrace_fork_parent.c: Ditto.
* testsuite/systemtap.clone/dtrace_vfork_parent.c: Ditto.
* testsuite/systemtap.clone/Makefile.fork_exec: Removed unused define.
* testsuite/systemtap.clone/Makefile.vfork_exec: Ditto.