[vcpkg baseline][libao] Disable dlfcn check under windows

[Cheney-W]

Describe the pull request
In an internal CI test, libao installation failed with following errors:

.././../src/1.2.2-d87489a217.clean/src/audio_out.c(291): error C2065: 'DIR': undeclared identifier
.././../src/1.2.2-d87489a217.clean/src/audio_out.c(291): error C2065: 'plugindir': undeclared identifier
.././../src/1.2.2-d87489a217.clean/src/audio_out.c(301): error C2037: left of 'd_name' specifies undefined struct/union 'dirent'
.././../src/1.2.2-d87489a217.clean/src/audio_out.c(301): error C2198: 'strlen': too few arguments for call
.././../src/1.2.2-d87489a217.clean/src/audio_out.c(301): error C2057: expected constant expression
.././../src/1.2.2-d87489a217.clean/src/audio_out.c(301): error C2466: cannot allocate an array of constant size 0

This issue only occurs when dlfcn-win32 installed before libao in Windows. Libao always check dlfcn.h and enable the macro HAVE_DLOPEN if dlfcn.h is exists, then the DIR struct will be compiled, in windows environment this will cause the above errors to appear.

For fixing this issue, I passed a cache variable to disable the search of dlfcn under windows

No feature need to be tested.

[dg0yt]

Does it need a patch, or could you just pass a corresponding cache vale (ac_cv_...=no or similar) to configure? (See config.log... for ac_cv_... variables resulting from detection.)

[Cheney-W]

@dg0yt Thanks for your guidance!

[JackBoosY]

LGTM if the tests passed.

[JackBoosY]

@vicroms Please consider to merge this PR first.

[dg0yt]

This is bad CMake style. It unsets the variable in the current scope, unhiding a possible value from a parent scope. For a clean variable, always use:

set(VAR "")

[dg0yt]

https://github.com/microsoft/vcpkg/blob/master/docs/maintainers/cmake-guidelines.md