On macOS, since Big Sur, the libraries were moved to a cache. The SDK comes
with stubs in the SDK (`/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.1.sdk/usr/lib/`
or whatever SDK version one has installed) where most have the `.tbd` suffix
(although some still are `.a`). Forcing `CMAKE_FIND_LIBRARY_SUFFIXES` on Apple
platforms broke building, unless one has copies of the libraries installed
elsewhere (like Brew), as many libraries (like `iconv` or `pcre`) are not
found.

This fix disables setting the `CMAKE_FIND_LIBRARY_SUFFIXES` to `.a` if
the platform is `APPLE` when building static libs.