travis: add job building wlroots without various options

Recently, the support for building rootston without XWayland was broken.
While the breakage was easily fixable, the more important problem is
that actually nobody seems to ever compile wlroots without XWayland,
causing silent breakage. This problem can easily be handled by the CI
system: in addition to the existing job which enables XWayland, add a
second job which compiles wlroots without XWayland. This will catch
breakage at a much earlier stage.

Catching broken builds without XWayland support obviously only covers
one part of a greater issue, as all the other options may cause our
builds to break, too. While we cannot test each combination, we can at
least make sure to have one build with all options enabled and one with
all options disabled.

.travis.yml

@@ -6,6 +6,12 @@ compiler:
   - gcc
   - clang
 
+# Settings to try
+env:
+ matrix:
+  - OPTIONS="-Denable_libcap=true  -Denable_systemd=true  -Denable_elogind=false -Denable_xwayland=true"
+  - OPTIONS="-Denable_libcap=false -Denable_systemd=false -Denable_elogind=false -Denable_xwayland=false"
+
 arch:
   packages:
     - meson
@@ -19,7 +25,7 @@ arch:
     - xcb-util-image
     - libcap
   script:
-    - "meson build"
+    - "meson build $OPTIONS"
     - "ninja -C build"
 
 script: