BUILD_TESTING is default enabled, which is a good default anyway.
Move the CppUnit check to the root CMakeLists.txt, simpler and clearer.
BUILD_TESTS is obsolete.
The need for BUILD_SHARED_LIBS=OFF for testing is not clear, it works on Linux.
But I kept it in the instructions for now.
Signed-off-by: Andreas Sturmlechner <asturm@gentoo.org>