Use ninja if available to build dependencies

If the ninja build system is installed, ask CMake to generate Ninja build files and use
it for the build.

README.md

@@ -75,6 +75,10 @@ open-source libraries on which **MoltenVK** relies:
 		brew install cmake
 		brew install python3
 
+   For faster dependencies build, you can also install Ninja:
+
+		brew install ninja
+
 2. Clone the `MoltenVK` repository:
 
 		git clone https://github.com/KhronosGroup/MoltenVK.git

fetchDependencies

@@ -48,8 +48,13 @@ build_repo() {
 
 	mkdir -p $1/build
 	cd $1/build
-	cmake ..
-	make
+	if type ninja >/dev/null 2>&1 ; then
+		cmake .. -G Ninja
+		ninja
+	else
+		cmake ..
+		make
+	fi
 	cd -
 }