Use CMake's Ninja backend

The difference between Make and Ninja for full, non-incremental builds
isn't that much [1,2]; but Ninja is already being used by Meson, so
why not?

[1] https://david.rothlis.net/ninja-benchmark/
[2] http://hamelot.io/programming/make-vs-ninja-performance-comparison/

https://gitlab.gnome.org/GNOME/nautilus/merge_requests/386

build-aux/flatpak/org.gnome.Nautilus.json

@@ -33,7 +33,7 @@
                 "-DCMAKE_INSTALL_PREFIX=/app",
                 "-DCMAKE_INSTALL_LIBDIR=/app/lib"
             ],
-            "buildsystem" : "cmake",
+            "buildsystem" : "cmake-ninja",
             "sources" : [
                 {
                     "type" : "archive",

build-aux/flatpak/org.gnome.Nautilus.yml

@@ -29,7 +29,7 @@ modules:
   config-opts:
   - "-DCMAKE_INSTALL_PREFIX=/app"
   - "-DCMAKE_INSTALL_LIBDIR=/app/lib"
-  buildsystem: cmake
+  buildsystem: cmake-ninja
   sources:
   - type: archive
     url: https://github.com/Exiv2/exiv2/archive/v0.26.tar.gz

build-aux/flatpak/org.gnome.NautilusGtk4.yml

@@ -29,7 +29,7 @@ finish-args:
 
 modules:
   - name: "exiv2"
-    buildsystem: "cmake"
+    buildsystem: "cmake-ninja"
     sources:
       - type: "archive"
         url: "https://github.com/Exiv2/exiv2/archive/v0.26.tar.gz"

build-aux/flatpak/org.gnome.NautilusMaster.yml

@@ -29,7 +29,7 @@ finish-args:
 
 modules:
   - name: "exiv2"
-    buildsystem: "cmake"
+    buildsystem: "cmake-ninja"
     sources:
       - type: "git"
         url: "https://github.com/Exiv2/exiv2.git"