Various minor cleanups.

Details:
- Rewrote bli_winsys.c to define bli_setenv() and bli_sleep()
  unconditionally, but differently for Windows and non-Windows, but
  then disabled the definition of bli_setenv() entirely since BLIS
  no longer needs to set environment variables. Updated bli_winsys.h
  accordingly, and call bli_sleep() from within testsuite instead of
  sleep() directly.
- Use
    #if !defined(_POSIX_BARRIERS) || (_POSIX_BARRIERS != 200809L)
  instead of
    #if !defined(_POSIX_BARRIERS) || (_POSIX_BARRIERS < 0)
  when guarding against local definition of pthread barrier in
  testsuite. (The description for unistd.h implies that _POSIX_BARRIERS
  should always be set to 200809L when barriers are supported, though I
  won't be surprised if we encounter a case in the future where it is
  set to something else such as 1 while still supported.)
- Removed old _VERS_CONF_INST definitions and installation rules in
  top-level Makefile. These are no longer needed because we no longer
  output libraries with the version and configuration name as
  substrings.
- Comment/whitespace updates in Makefile, config.mk.in, common.mk,
  configure, bli_extern_defs.h, and test_libblis.h.
- Added mention of 1m to README.md and other trivial tweaks.

frame/thread/bli_thread.c

@@ -1227,7 +1227,7 @@ void bli_thread_set_env( const char* env, dim_t value )
 	// Set the environment variable using the string we just wrote to via
 	// sprintf(). (The 'TRUE' argument means we want to overwrite the current
 	// value if the environment variable already exists.)
-	r_val = setenv( env, value_str, TRUE );
+	r_val = bli_setenv( env, value_str, TRUE );
 
 	// Check the return value in case something went horribly wrong.
 	if ( r_val == -1 )