Travis builds with system SWIG, HDF5 and FFTW3

[paskino]

I noticed that in the .travis.yml we use
EXTRA_BUILD_FLAGS="-DUSE_SYSTEM_SWIG=ON -DUSE_SYSTEM_HDF5=ON -DUSE_SYSTEM_FFTW3=ON

Is that correct? Should not we use the SWIG, HDF5 and FFTW3 that are built as part of the SuperBuild? Or these settings do not do anything?

[KrisThielemans]

yes, the plan was to use a mixture for the different builds, but at present it always uses those options. Serious work on this needs sorting out of the Travix matrix. see #17.

Note that building all the external packages can take a long time, making the travis builds too long and even time-out. Maybe there's a way to cache those (but that seems non-trivial)

[casperdcl]

ccache?

also note we can use https://docs.travis-ci.com/user/docker/ so perhaps cache docker layers.

and of course travis_wait.