configure: add libdl detection to ladspa, vf_dlopen
options: add --cache-default option

Add this option, which lets users set the cache size without forcing it
even when playing from the local filesystem.

Also document the default value explicitly.

The Matroska linked segments case is slightly simplified: they can
never come from network (mostly because it'd be insane, and we can't
even list files from network sources), so the cache will never be
enabled automatically.
cocoa_common: change video size when changing screen DPI

This improves the situation when moving a window from screens with different
DPIs.
Fix loading external subtitles with libass
build: make the "built on" report opt-out
build: change vf_dlopen test

Didn't work on Windows. Apparently, WIN32 is not set in the Makefile.
stream: remove some more forgotten network stuff

Was not cleanly removed with internal network code removal.
m_struct: try to be more standard C

The main problem is that this m_struct stuff uses pointers for offsets
(why...), so we mangle it by intptr_t. This stuff really should use ints
(or in theory ptrdiff_t) for offsets, but changing it would be too much
effort, and hopefully this m_struct stuff will go away and replaced by
the common option parser mechanism instead.

Shuts up warnings on Windows.

Patch suggested by jon_y and rossy on IRC.
ao_wasapi0: use new mp_ring buffer
configure: fix vcd detection on Windows
stream_vcd: use intptr_t cast for _open_osfhandle in accordance to MSDN