Detection of static Qt builds will fail for OS X non-framework builds

https://codereview.qt-project.org/#change,53305 originally implemented this.

However, the code only checks .dll and .so files, neglecting extensions on other platforms like .dylib, .a and .sl.

To make this more portable we should list

*Core*.*

and utilize QLibrary::isLibrary() to isolate the file extensions checking. If isLibrary returns true for any of the returned items, static build should be set to false (e.g. is dynamic).

Otherwise it can be left at true (e.g. is static).

If the list contains no items at all, we should probably not assume static, and fail with an error.