Doc: Update links in INSTALL file

Documentation URLs no longer use module names in path. Also, the
pages discussing installation from source have been renamed.

Change-Id: I135dc527ec9170eab62b0ffd1f4a80cdd834e186
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

INSTALL

         INSTALLING Qt Source Package Version %VERSION%.
 
-For full installation instructions for each supported platform, please
-see http://qt-project.org/doc/qt-%SHORTVERSION%/qtdoc/installation.html,
+For instructions on building and installing Qt for each supported platform,
+please see http://qt-project.org/doc/qt-%SHORTVERSION%/build-sources.html,
 or follow one of these links:
 
-Mac OS X:         http://qt-project.org/doc/qt-%SHORTVERSION%/qtdoc/install-mac.html
-Windows:          http://qt-project.org/doc/qt-%SHORTVERSION%/qtdoc/install-win.html
-Windows CE:       http://qt-project.org/doc/qt-%SHORTVERSION%/qtdoc/install-wince.html
-X11 Platforms:    http://qt-project.org/doc/qt-%SHORTVERSION%/qtdoc/install-x11.html
+Mac OS X:         http://qt-project.org/doc/qt-%SHORTVERSION%/macosx-building.html
+Windows:          http://qt-project.org/doc/qt-%SHORTVERSION%/windows-building.html
+X11 Platforms:    http://qt-project.org/doc/qt-%SHORTVERSION%/linux-building.html
+Windows CE:       http://qt-project.org/doc/qt-%SHORTVERSION%/install-wince.html