build: utilize built-in gnu_dirs module for installation directories

This started breaking with newer (2.0.x) waf versions, and it was
noticed that a built-in waf module was providing very similar
functionality.

APPNAME definition was required to have `gnu_dirs`' PACKAGE
variable to be defined in order to have f.ex. documentation
installed to the correct directory.

Currently unused options added by `gnu_dirs` were removed to clean
up the output of the help command.

Effective changes to behavior:
- `gnu_dirs` will attempt to figure out if it needs to use lib64
  instead of lib within your installation prefix. If you would
  like it to not do that, set `--libdir` during configuration.
  The way it tries to figure lib/lib64 out is if there's a
  `/usr/lib64` and no `/usr/lib32`.
- `--incdir` is now `--includedir` as per standard `gnu_dirs`
  behavior.

wscript_build.py

@@ -693,7 +693,7 @@ def build(ctx):
             features     = 'subst',
             PREFIX       = ctx.env.PREFIX,
             LIBDIR       = ctx.env.LIBDIR,
-            INCDIR       = ctx.env.INCDIR,
+            INCDIR       = ctx.env.INCLUDEDIR,
             VERSION      = libversion,
             PRIV_LIBS    = get_deps(),
         )
@@ -701,7 +701,7 @@ def build(ctx):
         headers = ["client.h", "qthelper.hpp", "opengl_cb.h", "render.h",
                    "render_gl.h", "stream_cb.h"]
         for f in headers:
-            ctx.install_as(ctx.env.INCDIR + '/mpv/' + f, 'libmpv/' + f)
+            ctx.install_as(ctx.env.INCLUDEDIR + '/mpv/' + f, 'libmpv/' + f)
 
         ctx.install_as(ctx.env.LIBDIR + '/pkgconfig/mpv.pc', 'libmpv/mpv.pc')