[Discuss-gnuradio] Close to tagging and releasing 3.5.3, merging next branch

[Johnathan Corgan]

At this point, the master branch of the repository is ready for
tagging and releasing as 3.5.3, which is the last planned release of
the 3.5.x series.  It will have a maint branch, so there is a
possibility of a 3.5.3.x bug fix release, but no plans for a 3.5.4
with new features.

If there are minor patches or bugfixes you've been holding on to, now
is the time to submit them.

Shortly after tagging 3.5.3, we'll be merging the next branch back
into master, which will switch the master branch to 3.6.x.

The 3.6 API hasn't changed that much from 3.5, but some important
structural changes have occurred that may create problems for people
who routinely pull from master:

- The autotools build system has been removed, and CMake has become
the official build system.  Build scripts will need to be adjusted to
account for this.

- All scripts that get installed into $PATH have had their file
extensions stripped.  So for example, scripts that call
uhd_rx_cfile.py will need to be changed to call uhd_rx_cfile without
the extension.

There will be more documented about actual feature changes in 3.6 on the wiki.

Johnathan