Travis: Default to -fno-common

See #3914

.travis.yml

@@ -31,7 +31,7 @@ install:
 script:
   - docker run -v $PWD:/usr/src/i3/ -w /usr/src/i3 ${BASENAME} ./travis/check-safe-wrappers.sh
   - docker run -v $PWD:/usr/src/i3/ -w /usr/src/i3 ${BASENAME} ./travis/check-formatting.sh
-  - docker run -v $PWD:/usr/src/i3/ -w /usr/src/i3 -e CC ${BASENAME} /bin/sh -c 'autoreconf -fi && mkdir -p build && cd build && (../configure || (cat config.log; false)) && make -j CFLAGS="-Wformat -Wformat-security -Wextra -Wno-unused-parameter -Wstrict-prototypes -Wmissing-prototypes -Werror"'
+  - docker run -v $PWD:/usr/src/i3/ -w /usr/src/i3 -e CC ${BASENAME} /bin/sh -c 'autoreconf -fi && mkdir -p build && cd build && (../configure || (cat config.log; false)) && make -j CFLAGS="-Wformat -Wformat-security -Wextra -Wno-unused-parameter -Wstrict-prototypes -Wmissing-prototypes -Werror -fno-common"'
   - docker run -v $PWD:/usr/src/i3/ -w /usr/src/i3 ${BASENAME} ./travis/check-spelling.pl
   - docker run -v $PWD:/usr/src/i3/ -w /usr/src/i3 -e CC ${BASENAME} ./travis/run-tests.sh
   - ./travis/skip-pkg.sh || docker run -v $PWD:/usr/src/i3/ -w /usr/src/i3 ${BASENAME} ./travis/debian-build.sh deb/debian-amd64/DIST