Configure: rebuild perl module nginx.so if headers are changed.

Note: the "-p" argument of cp(1) dropped intentionally, to force nginx.so
rebuild.  It is considered too boring to properly list all dependencies
in Makefile.PL.


git-svn-id: svn://svn.nginx.org/nginx/trunk@5071 73f98a42-aea0-e011-b76d-00259023448c