Implemented library self-initialization.

Details:
- Defined two new functions in bli_init.c: bli_init_once() and
  bli_finalize_once(). Each is implemented with pthread_once(), which
  guarantees that, among the threads that pass in the same pthread_once_t
  data structure, exactly one thread will execute a user-defined function.
  (Thus, there is now a runtime dependency against libpthread even when
  multithreading is not enabled at configure-time.)
- Added calls to bli_init_once() to top-level user APIs for all
  computational operations as well as many other functions in BLIS to
  all but guarantee that BLIS will self-initialize through the normal
  use of its functions.
- Rewrote and simplified bli_init() and bli_finalize() and related
  functions.
- Added -lpthread to LDFLAGS in common.mk.
- Modified the bli_init_auto()/_finalize_auto() functions used by the
  BLAS compatibility layer to take and return no arguments. (The
  previous API that tracked whether BLIS was initialized, and then
  only finalized if it was initialized in the same function, was too
  cute by half and borderline useless because by default BLIS stays
  initialized when auto-initialized via the compatibility layer.)
- Removed static variables that track initialization of the sub-APIs in
  bli_const.c, bli_error.c, bli_init.c, bli_memsys.c, bli_thread, and
  bli_ind.c. We don't need to track initialization at the sub-API level,
  especially now that BLIS can self-initialize.
- Added a critical section around the changing of the error checking
  level in bli_error.c.
- Deprecated bli_ind_oper_has_avail() as well as all functions
  bli_<opname>_ind_get_avail(), where <opname> is a level-3 operation
  name. These functions had no use cases within BLIS and likely none
  outside of BLIS.
- Commented out calls to bli_init() and bli_finalize() in testsuite's
  main() function, and likewise for standalone test drivers in 'test'
  directory, so that self-initialization is exercised by default.

test/exec_sizes/test_size.c

@@ -58,7 +58,7 @@ int main( int argc, char** argv )
 	int   ii;
 
 #ifdef NBLIS
-	bli_init();
+	//bli_init();
 #endif
 
 
@@ -533,7 +533,7 @@ int main( int argc, char** argv )
 	}
 
 #ifdef NBLIS
-	bli_finalize();
+	//bli_finalize();
 #endif
 
 	return 0;