Add -Wno-cast-function-type to suppress GCC8 warnings

See nodejs/nan#807

Author: zbjornson

binding.gyp

@@ -136,6 +136,7 @@
             '<!@(pkg-config pangocairo --cflags-only-I | sed s/-I//g)',
             '<!@(pkg-config freetype2 --cflags-only-I | sed s/-I//g)'
           ],
+          'cflags': ['-Wno-cast-function-type'],
           'cflags!': ['-fno-exceptions'],
           'cflags_cc!': ['-fno-exceptions']
         }],