Ensure QMAKE_RFLAGSDIR is set correctly

This patch ensures that additional runpaths passed to configure via
the -R switch are added to the QMAKE_RFLAGSDIR variable.

Previously, although runpaths provided in this way were appended to the
linker options when building Qt itself, they were not appended to
the QMAKE_RFLAGSDIR value written to mkspecs/qconfig.pri.  This meant
that the DT_RPATH attribute was set incorrectly in binaries built from
projects other than Qt itself.

-----------------------------------------------------------------------
Configure flags             Expected value        Value before this fix
-----------------------------------------------------------------------
<none>                      DEFAULT_RPATH         DEFAULT_RPATH
-no-rpath                   <empty>               <empty>
-R X                        DEFAULT_RPATH:X       DEFAULT_RPATH
-no-rpath -R X              X                     <empty>
-prefix Y                   Y/lib                 Y/lib
-prefix Y -no-rpath         <empty>               <empty>
-prefix Y -R X              Y/lib:X               Y/lib
-prefix Y -no-rpath -R X    X                     <empty>
-----------------------------------------------------------------------

DEFAULT_RPATH = /usr/local/Qt-${QT_VERSION}/lib

Change-Id: Iaf1809b528ebd249694cf41e004173e881ca48ad
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>