Use CMAKE_CXX_COMPILER CMake variable to find Clang compiler

[laudrup]

Since the appveyor image doesn't seem to have the LLVM toolset install
with the Visual Studio 2019 image, but does have LLVM installed, use
the CMake variable for specifying the C++ compiler to find Clang.

The requires calling the Visual Studio batch file for setting the
correct paths to toolsets etc.

[codecov]

Codecov Report

Merging #18 (2992109) into master (62219b7) will decrease coverage by 1.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
- Coverage   85.52%   84.34%   -1.18%     
==========================================
  Files          14       14              
  Lines         691      722      +31     
==========================================
+ Hits          591      609      +18     
- Misses        100      113      +13     

Impacted Files	Coverage Δ
include/boost/wintls/detail/win32_file.hpp	57.14% <0.00%> (-15.59%)	⬇️
include/boost/wintls/detail/win32_crypto.hpp	73.07% <0.00%> (-9.54%)	⬇️
include/boost/wintls/detail/uuid.hpp	81.25% <0.00%> (-4.47%)	⬇️
...ude/boost/wintls/detail/cryptographic_provider.hpp	56.25% <0.00%> (-3.75%)	⬇️
include/boost/wintls/detail/context_impl.hpp	86.51% <0.00%> (-3.73%)	⬇️
...clude/boost/wintls/detail/async_handshake_impl.hpp	78.84% <0.00%> (-2.79%)	⬇️
include/boost/wintls/detail/sspi_impl.hpp	92.18% <0.00%> (+0.15%)	⬆️
include/boost/wintls/detail/async_write_impl.hpp	90.47% <0.00%> (+0.47%)	⬆️
include/boost/wintls/stream.hpp	81.33% <0.00%> (+0.51%)	⬆️
...nclude/boost/wintls/detail/async_shutdown_impl.hpp	64.51% <0.00%> (+1.18%)	⬆️
... and 2 more

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