For some reasons, the checkout on travis may not have any tags, so
`git describe` fails, so bootstrap fails.
* .travis.yml: If git describe fails, install some tag.
We have failures on Flex output, which are probably related to an old
release. Let's check.
In file included from src/scan-code-c.c:3:
src/scan-code.c:2198:21: error: empty paragraph passed to '@param' command
[-Werror,-Wdocumentation]
* @param line_number
~~~~~~~~~~~~~~~~~^
* .travis.yml: here.
Running all these tests might be overkill: it is very long, and don't
need full portability checks. Besides, some tests under Valgrind are
too slow and get killed by the CI (timeout of 10min without output).
* .travis.yml: here.
