Set PREFIX back to /usr/gnuradio, because systems that do not use /opt

Set PREFIX back to /usr/gnuradio, because systems that do not use /opt
and follow traditional UNIX partitioning will then put the build in /
(which does not have adequate space and violates the point of / being
separate).  The prefix /usr/gnuradio is also not known to be in use by
any packaging system, and it is important that test builds not
interfere with files controlled by packaging systems.  It may be that
OS-specific overrides are needed to choose a reasonable prefix on
systems where /usr/gnuradio is not appropriate.

Add usrp as a dependency for gr-radar-mono.  Now only expected
failures occur on NetBSD/i386 4.0ish (gr-audio-{alsa,osx,windows} and
gr-comedi).

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@7897 221aa14e-8319-0410-a670-987f0aec2ac5