Compilation of PACKAGE target fails if Wix Toolset is not installed on system.

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Describe the bug
When building using the ci/appveyor/build_library.bat script, the compilation fails with the following error message:

CPack: Create package using WIX
CPack: Install projects
CPack: - Install project: ShellAnything
CPack: Create package
CMake Error at C:/Program Files/CMake/share/cmake-3.10/Modules/CPackWIX.cmake:299 (message):
  Could not find the WiX candle executable.

To Reproduce
Steps to reproduce the behavior:

1. Build INSTALL target using ci/appveyor/build_library.bat script.

Expected behavior
The CMake script should not try to build the Wix Installer if Wix Toolset is not installed on the system. At worst, a warning should be displayed.

Screenshots
See the following attached log file issue 68.txt for details.

Environment

• OS: Windows 7
• Version: HEAD, 0.5.0, eadf63b

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Related to #61.

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Implemented in 23cae72.