Usually when microphone example is run you never hit buffer underflow and "fill with zeros" branch is never executed. But when it's executed it fails with "invalid value" because of checks performed on frame_count. This platform-independent check 8094dc5249/src/soundio.c (L447) sometimes passed because unitialized local frame_count have arbitrary value, but this CoreAudio one 8094dc5249/src/coreaudio.c (L1078) is not passed, and "fill with zeros" scenario always fails on MacOS.
- Consistent with usage of `libsoundio_SOURCE_DIR`
- Makes it easier to include inside other projects.
I haven't encountered any actual issues with the way it is now, but it
was misleading while I was debugging a different issue related to being
used as a subproject.
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.
This helps alleviate cases where compilers will choose to link against the dynamic library instead of the static based on name alone (in which you don't have much control, or don't wish to tinker too much with the compiler args)
When compiling with Visual Studio (#49), prevents this error:
```
1>C:\Program Files (x86)\Windows Kits\10\Include\10.0.10586.0\um\commdlg.h(929): error C2373: 'IPrintDialogCallbackVtbl': redefinition; different type modifiers
1> C:\Program Files (x86)\Windows Kits\10\Include\10.0.10586.0\um\commdlg.h(929): note: see declaration of 'IPrintDialogCallbackVtbl'
1>C:\Program Files (x86)\Windows Kits\10\Include\10.0.10586.0\um\commdlg.h(961): error C2373: 'IPrintDialogServicesVtbl': redefinition; different type modifiers
1> C:\Program Files (x86)\Windows Kits\10\Include\10.0.10586.0\um\commdlg.h(961): note: see declaration of 'IPrintDialogServicesVtbl'
```
# 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.
