CI: make sure `git describe` works

[Akim Demaille]

commit ccc3a92aaefd275a8306f032a9055879fe1879b9
Author: Akim Demaille <address@hidden>
Date:   Sat Sep 15 18:57:44 2018 +0200

   CI: make sure `git describe` works

   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.

diff --git a/.travis.yml b/.travis.yml
index 856c200c..da09fb99 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -186,6 +186,12 @@ before_install:
  - eval "${MATRIX_EVAL}"

script:
+  - git show
+  - git tag -l
+  # For some reasons, sometimes the checkout does not have any tags,
+  # so `git describe` fails, so bootstrap fails.
+  - git describe || git tag v3.0 -m "Fake version 3.0."
+  - git describe
  - ./bootstrap
  - ./configure --enable-gcc-warnings CC="$CC" CXX="$CXX" || cat config.log
  - make -j2