fltk: fix build

The build of fltk was failing due to an issue in the usage of
scandir(). This issue is fixed by a new patch,
fltk-1.1.7-scandir-fix.patch. Moreover, this problem was not
necessarly visible, as the build process of FLTK didn't bail out with
an error when it was failing. This if fixed by
fltk-1.1.7-bail-out-on-error.patch. Finally, there was a problem with
the tests of FLTK, so we disabled them.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>