libcodec2: bump to 0.7, switch to CMake

Since version 0.6, libcodec2 switched buildsystem from autotools to
cmake.

The patches in the libcodec2 directory were removed because they were
only relevant to the autotools build, but another patch was added to
fix the codec2.pc file being installed.

For the cmake package unittests are disabled by default. The download
site is changed to http://files.freedv.org/codec2

. An option is added
to be able to install examples to target.

Signed-off-by: Olivier Schonken <olivier.schonken@gmail.com>
[Thomas: switch to 0.7, add patch to fix .pc file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>