ci: make Meson warnings fatal

New warnings can be hard to notice in CI, since CI will just pass in
that case. Meson sometimes uses warnings for important mistakes, e.g.
invalid option.

Let's turn warnings into errors so that we can spot these more easily.

.builds/alpine.yml

@@ -30,7 +30,7 @@ tasks:
       sudo ninja -C build install
   - setup: |
       cd sway
-      meson build -Dauto_features=enabled -Dtray=disabled
+      meson build --fatal-meson-warnings -Dauto_features=enabled -Dtray=disabled
   - build: |
       cd sway
       ninja -C build
@@ -43,5 +43,5 @@ tasks:
       mkdir subprojects
       ln -s ../../wlroots subprojects/wlroots
       rm -rf build
-      meson build --default-library=static --force-fallback-for=wlroots
+      meson build --fatal-meson-warnings --default-library=static --force-fallback-for=wlroots
       ninja -C build

.builds/archlinux.yml

@@ -27,7 +27,7 @@ tasks:
       sudo ninja -C build install
   - setup: |
       cd sway
-      meson build -Dauto_features=enabled -Dsd-bus-provider=libsystemd
+      meson build --fatal-meson-warnings -Dauto_features=enabled -Dsd-bus-provider=libsystemd
   - build: |
       cd sway
       ninja -C build

.builds/freebsd.yml

@@ -34,7 +34,7 @@ tasks:
     cd subprojects
     ln -s ../../wlroots wlroots
     cd ..
-    meson build -Dtray=enabled -Dsd-bus-provider=basu
+    meson build --fatal-meson-warnings -Dtray=enabled -Dsd-bus-provider=basu
 - build: |
     cd sway
     ninja -C build