Add exported/installed CMake target

[CJKay]

This commit adds the WIL CMake interface library, which allows the
project to be exported and installed for consumption by other CMake
projects in the standard fashion.

The logic to create the NuGet package has been replaced with the
built-in CPack NuGet support introduced with CMake 3.13, which
automatically packages any files and directories installed by the
target. This replaces the make_wil_nupkg custom target with the
standard CPack package target, which builds the package with all the
configured package generators (for now that's just NuGet).

Note that the original NuGet specification file used the new license
style, whereas CPack still generates a NuGet specification with the
deprecated style. This will hopefully be resolved by future versions of
CPack.

Resolves #118.

[msftclas]

All CLA requirements met.

[dunhor]

It seems like CMake is failing to initialize on the build servers that create and sign the NuGet package. Here's the output:

-- The CXX compiler identification is MSVC 19.16.27038.0
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2017/Enterprise/VC/Tools/MSVC/14.16.27023/bin/Hostx86/x86/cl.exe
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2017/Enterprise/VC/Tools/MSVC/14.16.27023/bin/Hostx86/x86/cl.exe -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
CMake Error at C:/Program Files (x86)/Microsoft Visual Studio/2017/Enterprise/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.12/Modules/WriteBasicConfigVersionFile.cmake:30 (message):
  Unknown keywords given to WRITE_BASIC_CONFIG_VERSION_FILE():
  "ARCH_INDEPENDENT"
Call Stack (most recent call first):
  C:/Program Files (x86)/Microsoft Visual Studio/2017/Enterprise/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.12/Modules/CMakePackageConfigHelpers.cmake:211 (write_basic_config_version_file)
  CMakeLists.txt:45 (write_basic_package_version_file)

The only thing that stands out is that it seems that these build machines are still using VS 2017 (something out of our control, unfortunately).

[ChrisGuzak]

looks like goodness but I'm not CMake savvy enough to read this. @dunhor can you check this as I understood you created the cmake and nuget build projects?

[dunhor]

There were earlier problems with building the changes on the signing machines (see earlier comment of mine). I haven't checked recently, but I assume those issues still exist. Additionally, #137 from a while ago appears to have addressed the same problem, so there's likely some conflict resolution needed at the very least

[CJKay]

Apologies for abandoning this patch on your lap, I've been unable to find the time recently to ensure it's kept up to date.