minor prefix problem with 2.1.1 git

[Greg Woods]

I tried to use a non-default prefix, and it craps out on "make install"
because one of the sql-related files refuses to install in a directory
name that didn't end with /usr/local/lib .  I was able to work around
this by using a --prefix like "/local/freeradius-git/usr/local" instead
of just "/local/freeradius-git". A minor annoyance but I thought others
might want to hear about it. At least the error message was clear enough
that coming up with the workaround was easy.

--Greg