Require GTK+ version >= 2.16

Require GTK+ version >= 2.16

I'm got bug in my system:

lcc: "scale.c", line 374: warning: function "gtk_scale_add_mark" declared
          implicitly [-Wimplicit-function-declaration]
      gtk_scale_add_mark (GTK_SCALE (widget), 0.0, GTK_POS_LEFT, NULL);
      ^

  CCLD   scale
scale.o: In function `app_new':
(.text+0x23e4): undefined reference to `gtk_scale_add_mark'
scale.o: In function `app_new':
(.text+0x250c): undefined reference to `gtk_scale_add_mark'
scale.o: In function `app_new':
(.text+0x2634): undefined reference to `gtk_scale_add_mark'
make[2]: *** [scale] Error 1
make[2]: Target `all' not remade because of errors.

$ pkg-config --modversion gtk+-2.0
2.12.1

The demos/scale.c use call to gtk_scale_add_mark() function from 2.16+
version of GTK+. Need do support old GTK+ (rewrite scale.c) or simple
demand of high version of GTK+, like this: