mkrockspecs: set datarootdir in addition to datadir subset.

* src/mkrockspecs.in (default.build): When using a GNU autotools
driven installation, we still set datarootdir so that (among
others) the documentation trees are installed to the correct rock
directory.  But, don't mangle datadir while doing so!

Signed-off-by: Gary V. Vaughan <gary@gnu.org>