use vcpkg only for *-msvc targets

[ognevny]

for other windows targets (*-gnu and *-gnullvm) pkg-config is usually used

[ognevny]

I've rebased branch and simplified patch a bit
@alexcrichton can this be merged? I see other -sys crates use vcpkg for MSVC only

[alexcrichton]

I'm sorry but I don't personally have the energy to shepherd this. Changes like this unfortunately have a pretty high likelihood of causing breakage for users. That breakage typically requires follow-ups from the maintainership side here which I don't have a ton of time for nowadays. If someone else wants to champion this though and field requests though that's ok too! (they'll need to merge though)

[ognevny]

we can change it to run both vcpkg and pkg-config on windows-gnu. it shouldn't break "pure" msys2 environments (like CIs)