ruby: fix a parallel building issue

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Acked-by: Cliff Brake <cbrake@bec-systems.com>
ruby: fix race conditions at install-ext

* lib/mkmf.rb (create_makefile): fix race conditions at install-ext.
  target files need to depend on destination directory timestamp
  files, not phony trgets.

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Acked-by: Cliff Brake <cbrake@bec-systems.com>