modules: Fix FindWebP.cmake for windows

[Squareys]

Hi @mosra !

This is a very simple one, the precompiled binaries on windows are named libwebp.lib and similar, and on Windows, NAMES webp is therefore not enough, since windows has empty CMAKE_STATIC_LIBRARY_PREFIX.

Best,
Jonathan

[mosra]

Ah lol once again Google's "buildsystem" that doesn't care about platform differences and uses a lib prefix everywhere. Thanks!

Annoying request: since you say there's a prebuilt library, could you make this be built on AppVeyor as well? (I won't get mad if you refuse.)

[Squareys]

@mosra I did actually try for 15 Minutes, but because they package their prebuilt binaries with an additional folder inside the zip, I got stuck trying to unpack the zip such that the include, lib and bin would get merged into dependencies. Currently unfortunately that meat I had to give up, too much going on atm :/

[mosra]

Merged as 36c8af4, and in e28d05b I added the Windows CI build to have this covered there as well. Wasn't too bad in the end :P