CustomTarget: require install_dir only when install is true (#519)

mesonbuild/build.py

@@ -871,6 +871,7 @@ class CustomTarget:
             self.install = kwargs['install']
             if not isinstance(self.install, bool):
                 raise InvalidArguments('"install" must be boolean.')
+            if self.install:
                 if 'install_dir' not in kwargs:
                     raise InvalidArguments('"install_dir" not specified.')
                 self.install_dir = kwargs['install_dir']