[SUGGESTED FEATURE] USE_SVNREPO allows to build the port using only subversion repository URL

[Yuri]

I came across one package which doesn't distribute source tarballs, and 
only offers subversion URL. I implemented the new feature USE_SVNREPO 
that allows to build the port in such setup. Here is what it does:
* exports source from the subversion repository for specified URL/revision
* resets the timestamp of every file and directory to 1970-01-01 for 
reproducibility
* creates tarball

Patch adding this feature is attached to this bug report: 
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=198449

Yuri