- Fix build for Leap 42.3: Force use of GCC7.

- Update to version 1.6.0:
* Support for effects that work as compute shaders. Compute
shaders are generally slower than fragment shaders for the same
algorithm, but allow some forms of communication between shader
invocations and have more flexible output, which can enable
more efficient algorithms. See effect.h for more details. Note
that the fastest rendering API on EffectChain is now to a
texture if possible, not to an FBO. This will only matter if
the last effect is a compute shader.
* Movit now includes a compute shader implementation of
DeinterlaceEffect, which is automatically used instead of the
fragment shader implementation if your GPU and OpenGL driver
supports it (in practice, this means on all platforms except on
macOS). The compute shader version is typically 20–80% faster
than the fragment shader version, depending on your GPU and
other factors.
A compute shader implementation of ResampleEffect was written but
ultimately failed to be faster, and so is not included.
* Support for microbenchmarks of effects through the Google
microbenchmarking framework (optional). Currently,
DeinterlaceEffect and ResampleEffect has benchmarks; enable
them by running the unit test with --benchmark (also try
--benchmark --help).
* Effects can now explicitly request _not_ to have mipmaps, which
means they can do so without needing to request bounce and
fiddling with the sampler state. Note that this is an API
change for effects.
* Movit now requires C++11, both to build and to #include the