Add -archdatadir and change some of the default install dirs in Qt 5.

Add -archdatadir and change some of the default install dirs in Qt 5.

Architecture-depedent Qt data defaults now to something under
-archdatadir. Architecture-dependent data is everything that contains
machine code (e.g., plugins) as well as anything that hardcodes
build-specific data, like qconfig.pri and qmodule.pri. That is:

  QML imports:    $archdatadir/imports     (includes plugins)
  Qt plugins:     $archdatadir/plugins     (machine code)
  Mkspecs:        $archdatadir/mkspecs     (build-specific)

Architecture-independent Qt data defaults now to something under
-datadir. This option existed in Qt 4, but did not differentiate between
arch-dependent and independent. Following Autoconf's lead, --datadir is
the *independent* data root.

  translations:   $datadir/translations    (.qm files are arch-independent)
  docs:           $datadir/doc

By default, both new options are equal to the Qt install prefix.

(Strictly speaking, for complete Autoconf compatibility, we'd need a
--datarootdir=$prefix/share, --datadir=$datarootdir/qt5 and
--docdir=$datarootdir/doc/qt5, but that's just nitpicking and
unnecessary)

Change-Id: I39c886a6a2d2d2c0b11923c50974179e21f2af76
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>