Set dynamic library prefix to "lib" for MacOS in HostOsInfo.
Set dynamic library prefix to "lib" for MacOS in HostOsInfo.

This makes the definition consistent with mac-gcc.qbs, causing a failing
autotest to succeed.
(Frameworks do not use a prefix or suffix and will need special
handling.)

Change-Id: I8ef500895cfd43f71736c3313521dec70cc6b484
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@digia.com>

qt-labs/qbs,gatzka/qt-labs-qbs,villytiger/qbs,qt-labs/qbs,qt-labs/qbs,Distrotech/qbs,villytiger/qbs,villytiger/qbs,Distrotech/qbs,qtproject/qt-labs-qbs,villytiger/qbs,Distrotech/qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,Distrotech/qbs,Distrotech/qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,Distrotech/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,Distrotech/qbs,gatzka/qt-labs-qbs,villytiger/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,villytiger/qbs,qt-labs/qbs,qt-labs/qbs