[TRAVIS] Enable the build of apps.

[frozar]

More tests in Travis CI.

[taketwo]

That was on the edge of 70 minutes!

[frozar]

Continous integration is definitely an issue for PCL. Rework has to be done as we know.

[SergioRAgostinho]

The first build failed on the merge with master 🤦‍♂️

[frozar]

For the non regression tests, everything should run without VTK, shouldn't they?

[frozar]

On my repository, I created the branch travis_without_vtk on top of master. I just use -DWITH_VTK=OFF to disable vtk. You can see the really promissing results of Travis CI there:
https://travis-ci.org/frozar/pcl/builds/280901244

The build of doc failed (as usual on my repository) and the job test-ext-1 failed also with this output:

95% tests passed, 1 tests failed out of 21

Total Test time (real) =  85.76 sec

The following tests FAILED:
	  4 - io_grabbers (Failed)

@taketwo, @SergioRAgostinho: What do you think about it?

[taketwo]

Can we disable "build-apps", or at least some of the apps? It's annoying that every time a build fails I need to open Travis and check "ah, ok, it's build-apps task timed out as usual, no problem".

[SergioRAgostinho]

Since we're on the edge, why not try to just disable those 4 or 5 I enable explicitly.

As a side comment, last time I checked, refactoring the apps target was a real pain.