Removed duplicate CMake functionality

Removed duplicate CMake functionality

The FindXXX modules that I have removed are found in standard CMake
2.8.2, so we don't need to duplicate them if we choose to have a
similar minimum requirement for CMake. So far, I've arbitrarily chosen
2.8.0 as a compromise between modernity and widespread adoption.
See discussion in Redmine issue #815.

The logic for the work-around for Cray MPI is now in CMakeLists.txt

The test for the Git version has also moved into CMakeLists.txt

We had a minor change in the LibXml2 module that catered to some
weird/broken CMake environment Teemu had, but it looks to be
unnecessary for modern CMake.

The LAPACK find module is nearly the same as standard, apart from a
bug fix that we have found and reported upstream. So we have to
keep the duplicated code.

Change-Id: If312671392f9b77c0007c761ccb37688bcfc13b3