Travis: build mobile on Linux with newer gcc

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

.travis.yml

@@ -58,12 +58,14 @@ matrix:
     - env: SUBSURFACE_PLATFORM='linux2'
       os: linux
       dist: trusty
+      sudo: required
       language: c++
       addons:
         apt:
           sources:
             - sourceline: 'ppa:beineri/opt-qt-5.10.1-trusty'
               key_url: 'https://keyserver.ubuntu.com/pks/lookup?op=get&search=0x10C56D0DE9977759'
+            - ubuntu-toolchain-r-test
           packages:
             - qt510base
             - qt510connectivity
@@ -76,7 +78,8 @@ matrix:
             - qt510svg
             - qt510tools
             - git
-            - g++
+            - gcc-5
+            - g++-5
             - make
             - autoconf
             - automake

scripts/linux2/before_install.sh

@@ -8,6 +8,10 @@
 
 set -x
 
+# make sure we use a new enough compiler
+export CC=gcc-5
+export CXX=g++-5
+
 # Travis only pulls shallow repos. But that messes with git describe.
 # Sorry Travis, fetching the whole thing and the tags as well...
 git fetch --unshallow