find_package(ROOT) should include a set of default libraries

The standard way to using ROOT in CMake projects is by calling find_package(). The actual list of libraries is controlled by the COMPONENTS keyword. For example:

find_package(ROOT COMPONENTS Hist Tree RIO MathCore)

Will set ROOT_LIBRARIES to full path for libHist.so libTree.so libRIO.so libMathCore.so. To facilitate the life of developers perhaps it would be convenient to define a default set of libraries if the COMPONENTS keyword is not used. The set of libraries should be equivalent to root-config --libs.