AUDIOCLIENT_H, as declared here, is not an external cached variable, so calling mark_as_advanced() has no apparent effect (does not enable user to modify value, even in advanced mode). Even if it were cached, its value (success/failure at finding audioclient.h) should not be edited by the user.
Also modified whitespace to be more consistent with that found in CMakeLists.txt.
# Current issue:
When attempting to compile under MSVC (#49), CMake can't find the WASAPI headers. I'm running Windows 10, Visual Studio Community 2015, and the latest SDK.
# Proposed fix:
Use `check_include_file` instead of `find_path`, which seems more semantically correct anyways.
