/bin/build.sh: search for ninja and use it by default

bin/build.sh

@@ -1,7 +1,12 @@
 #!/bin/env sh
 
-if [ "$1" = "web" ]; then
-    emcmake cmake -B .build-web "${@:2}" && cmake --build .build-web --parallel
-else
-    cmake -B .build "$@" && cmake --build .build --parallel
+# check whether ninja is around (you better start running)
+if [ -x "$(command -v ninja)" ]; then
+    generator="-G Ninja"
+fi
+
+if [ "$1" = "web" ]; then
+    emcmake cmake $generator -B .build-web "${@:2}" && cmake --build .build-web --parallel
+else
+    cmake $generator -B .build "$@" && cmake --build .build --parallel
 fi