Allow vcpkg overlays to be specified as CMake variables

[lilleyse]

Currently projects using cesium-native set VCPKG_OVERLAY_PORTS and VCPKG_OVERLAY_TRIPLETS as environment variables. This PR allows you to instead set them as regular CMake variables, like what we do for VCPKG_TRIPLET.

@jherico would this be a welcome change to bring back into https://github.com/jherico/ezvcpkg?

[jherico]

@lilleyse I'll investigate. I haven't actually done anything overlay specific with ezvcpkg or vcpkg in general yet, so I have little experience. This change seems straightforward enough though. If you want to create a PR I'll likely merge it quickly.

[kring]

@azrogers the compiler argument code in compiler.cmake shouldn't be uncommented, because compiler options added in that way will apply globally, but just to cesium-native libraries. This isn't so much a problem in cesium-native, because everything outside cesium-native is built by vcpkg, which will not be affected. But other projects that use cesium-native can and do have third-party libraries, and we don't want to mess with their compiler flags.

The cmake code that sets the warnings for just the cesium-native libraries is here:

cesium-native/cmake/macros/configure_cesium_library.cmake

Line 3 in f646271

target_compile_options(${targetName} PRIVATE /W4 /WX /wd4201 /bigobj /w45038 /w44254 /w44242 /w44191 /w45220)

[lilleyse]

@azrogers could you also revert the formatting changes in configure_cesium_library.cmake and compiler.cmake?