meson: Convert xwayland option to feature type

wlroots as a subproject now yields the xwayland meson option to its
parent. We need to match the type for this to work. This also adds
support for auto mode, where xwayland is used if present but no warning
is given otherwise.

.github/workflows/main.yml

@@ -13,7 +13,7 @@ jobs:
       matrix:
         CC: [ gcc, clang ]
         OS: [ "alpine:edge", "archlinux:base-devel" ]
-        xwayland: [ true, false ]
+        xwayland: [ enabled, disabled ]
     container: ${{ matrix.OS }}
     env:
       CC: ${{ matrix.CC }}
@@ -54,7 +54,7 @@ jobs:
         run: git clone https://gitlab.freedesktop.org/wlroots/wlroots.git subprojects/wlroots -b 0.16.0
       - name: Check for formatting changes
         run: |
-          meson build-clang-format -Dxwayland=true
+          meson build-clang-format -Dxwayland=enabled
           ninja -C build-clang-format clang-format-check
 
   scan-build:
@@ -73,5 +73,5 @@ jobs:
         run: git clone https://gitlab.freedesktop.org/wlroots/wlroots.git subprojects/wlroots -b 0.16.0
       - name: Run scan-build
         run: |
-          meson build-scan-build -Dxwayland=true
+          meson build-scan-build -Dxwayland=enabled
           ninja -C build-scan-build scan-build