CI: beware of time limits

* .travis.yml (GCC 8): Run only the part 1 of the tests.

.travis.yml

@@ -200,7 +200,8 @@ jobs:
         - CONFIGUREFLAGS='CFLAGS=-O1 CXXFLAGS=-O1'
         - PART=1
 
-    - name: "GCC 8"
+    # With glr2.cc, we run out of time with the full test suite.
+    - name: "GCC 8 part 1"
       stage: check
       os: linux
       dist: bionic
@@ -210,6 +211,7 @@ jobs:
       env:
         - CC=gcc-8
         - CXX=g++-8
+        - PART=1
 
     - name: "GCC 7"
       stage: check

README-hacking.md

@@ -563,13 +563,15 @@ re-run the tests, run:
 ## Docker
 
 Running old compilers is not very easy.  Docker can be used for some of
-them.  Have a look at .travis.yml for setups.  Move the tarball in /tmp and
-run, for instance:
+them.  Have a look at .travis.yml for setups.  Move the tarball in `/tmp`
+and run, for instance:
 
 ```
 docker run -v /tmp:/tmp -it ubuntu:xenial
 ```
 
+This way, the host and guest machines share `/tmp`.
+
 ### GCC 4.6
 On Ubuntu Xenial.