meson: minor QOL and logic tweaks

A few custom targets had some less than optimal names which created some
misleading "Generating custom-target-name with a custom command"
messages. Change those to be more descriptive/correct. In a few other
places, some checks were being done that could easily be
skipped/ignored in certain cases (like checking for windows-related
headers when gl-win32 isn't true). Also rearrange that to be smarter.
Finally, print some extra libplacebo messages for enabling/disabling
vo_gpu_next.

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