Set is_found in AppleFrameworks constructor

Set is_found in AppleFrameworks constructor, rather than overriding the
found() method, as other superclass methods may access is_found.

mesonbuild/dependencies/platform.py

@@ -33,8 +33,7 @@ class AppleFrameworks(ExternalDependency):
         for f in self.frameworks:
             self.link_args += ['-framework', f]
 
-    def found(self):
-        return mesonlib.is_osx()
+        self.is_found = mesonlib.is_osx()
 
     def get_version(self):
         return 'unknown'