Transform GNU-makefile tests to CMake

ROOT’s main test suite roottest has several tests that still use GNU-Makefiles. For Windows, to ease maintenance, and to get rid of a legacy tool called rmkdepend we need to transform them to CMake. Start with a simple one, e.g. root/tree/string/Makefile!

Expected Results

Less GNU-Makefiles, more consistent / complete CMake-based setup.

Requirements

Basic CMake and GNU make knowledge; there is nothing ROOT specific.

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