Fix 'make dist' on FreeBSD due to 'cp -r' following symlinks.

cp(1) -r follows symlinks on FreeBSD. POSIX allows -r to be
implementation-defined, while clearly defining -R to do what
I want and expect.
Merge branch 'maint'

* maint:
  Fix 'make dist' on FreeBSD due to 'cp -r' following symlinks.