Set QMAKE_DOCS_INSTALLDIR to $$[QT_INSTALL_DOCS] to allow overriding in .pro

Set QMAKE_DOCS_INSTALLDIR to $$[QT_INSTALL_DOCS] to allow overriding in .pro

Normally you want to pass -installdir $$[QT_INSTALL_DOCS] to qdoc by
default. However, if you want to force the generation of URL links to
the documentation, the option cannot be specified.

By setting the QMAKE_DOCS_INSTALLDIR variable in default_pre.prf a
project may override it at will, as for example Qt Creator would do.

Change-Id: Ib31f03acf4e8050cf2dd3aa33f3a10ed027f1df7
Reviewed-by: Casper van Donderen <casper.vandonderen@nokia.com>