qt4: add version 4.8.0

qt4: add version 4.8.0

Version 4.8.0 makes a few minor changes in the internal build system -
the following issues had to be worked around:

* The -qt-gif configure option has been removed. This is actually the
  default and has been for some time, so remove it from qt4.inc.
* The mkspecs have been refactored requiring us to copy our g++.conf
  file over the top of g++-unix.conf instead. Some modifications to this
  file were also necessary to remove some settings that are now in other
  conf files (and we don't modify those values in any case).
* The LD environment variable needs to be unset during configure, or
  else the configure script overrides our value of QMAKE_LINK which
  selects ${CXX} as our linker.
* QMAKE_CXX contains a reference to OE_QMAKE_CXX which the configure
  script does not expect and cannot expand and this results in webkit
  being disabled, so add a workaround for this.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>