Mac: Fix detection of dynamic libraries

Fix detection of dynamic libraries on Mac outside of a framework.

This is the issue reported by Drew in QTCREATORBUG-4212 where
$ ls /Users/andrew/local/qt/lib/Core
/Users/andrew/local/qt/lib/libQtCore.4.7.2.dylib
/Users/andrew/local/qt/lib/libQtCore.4.7.dylib
/Users/andrew/local/qt/lib/libQtCore.4.dylib
/Users/andrew/local/qt/lib/libQtCore.dylib
/Users/andrew/local/qt/lib/libQtCore.la
/Users/andrew/local/qt/lib/libQtCore.prl
/Users/andrew/local/qt/lib/libQtCore_debug.4.7.2.dylib
/Users/andrew/local/qt/lib/libQtCore_debug.4.7.dylib
/Users/andrew/local/qt/lib/libQtCore_debug.4.dylib
/Users/andrew/local/qt/lib/libQtCore_debug.dylib
/Users/andrew/local/qt/lib/libQtCore_debug.la
/Users/andrew/local/qt/lib/libQtCore_debug.prl

is not detected.