gnuradio: update 3.7.2.1 -> 3.7.3

Changelog:
http://gnuradio.org/redmine/projects/gnuradio/wiki/ChangeLogV3_7_3/

(Build tested.)
uhd: update 3.5.4 -> 3.7.0

Build tested (with gnuradio).
plan9port: modify the builder to use the INSTALL script.

plan9port ships with an INSTALL script. This commit modifies the
builder to use the script instead of a custom build script. The
commit also adds a patch to build fontsrv, which is otherwise
omitted from the build.
Do not fetch X11 build dependencies if building on darwin.