Fix rootext shared library Makefiles
The current Makefiles (1) do not follow the naming convention used by implicit GNU make rules, e.g. the CPP variable name is supposed to be the C pre-processor not the C++ compiler and (2) most of the content is repeated boilerplate to compile a shared library that integrates well with CERN ROOT.
This commit implicit GNU make rules where possible and also changes dependency generation to be actually useful. Additionally all whitespace errors detected by git (spaces before tabs and trailing whitespaces) are fixed. A nice side-effect of this is that fully parallel builds are now possible.