disabled all backends on Cairo
pixman also icks up gtk, disabling
Merge branch 'issue_345_bare_bones_cairo' into devel
PYTHONUSERBASE is now CMAKE_INSTALL_PREFIX installing in Externals led to subsequent packages not finding pip installed modules, because PYTHONPATH not set properly. And it didn't make sense to spread the python packages in so many places
Fixed issue where pytable wouldn't find numexpr because pip was using a 'weird' pythonuserbase var