Makefile.inc1: Silence an error in make packages

[otis]

Summary

When on a system without system without subversion installed,
make packages reported an error like:

eval: /usr/src: Permission denied

This commit tries to fix that.

[imp]

How does this compare with https://reviews.freebsd.org/D30990 which is similar. Same problem?

[emaste]

svnversion etc. serves no purpose in main any longer so this whole section should be reworked, but I think this change is fine for now (it doesn't make anything worse) and can be MFC'd.