setup.py: Use the detected Ninja and quote build_dir

The build_dir is an absolute path, so quote it.

We don't really need to use the absolute path to Ninja in git-update
since we pass it to subprocess.call which will do an shutil.which

common.py

@@ -45,10 +45,9 @@ def git(*args, repository_path='.'):
                                    stderr=subprocess.STDOUT).decode()
 
 def accept_command(commands):
-    """Search @commands and returns the first found absolute path."""
+    """Search @commands and returns the first found command"""
     for command in commands:
-        command = shutil.which(command)
-        if command:
+        if shutil.which(command):
             return command
 
     return None

setup.py

@@ -54,7 +54,7 @@ class GstBuildConfigurer:
             subprocess.check_call(
                 [sys.executable, meson, "../"] + self.args + self.get_configs(), cwd=build_dir)
             print("\nYou can now build GStreamer and its various subprojects running:\n"
-                  " $ ninja -C %s" % build_dir)
+                  " $ {} -C {!r}".format(ninja, build_dir))
         except subprocess.CalledProcessError:
             return False