command: adjust previous commit

Due to the current code structure, the "current" entry and the entry
which is playing can be different. This is probably silly, but still
try to mark the entries correctly.

Refs #1260.
build: check for mingw-w64 explicitly

And fail building if not any of MingW-w64 or POSIX are found. Obviously,
mpv needs one of those 2.
wscript: move down some less important checks
command: implement A-B loops

Probably needs to be polished a bit more. Also, might require a key
binding that can set/clear the loop points in a more intuitive way.

For now, something like this can be put into input.conf to use it:

ctrl+y set ab-loop-a ${time-pos}    # set A
ctrl+x set ab-loop-b ${time-pos}    # set B
ctrl+c set ab-loop-a no             # clear (mostly)

Fixes #1241.