Exploit FetchContent for downloading Google Test framework at configuration
time, instead of ExternalProject.

Manually add the FetchContent module that is added in CMake 3.11, for
allowing using its capabilities with older CMake versions.

As CppUtilities is a header-only library that never works as a stand-alone
application, it is better to leave to the including super-project the
possibility to fully decide what compilation flags have to be used and where.

Add option for allowing super-project including CppUtilities via
add_subdirectory to not enable CppUtilities headers and targets
installation.

[MISC] Prefix names of variables controlling what targets are configured with the name of the project.

This change was made for avoiding ambiguities when setting variables such as PROJECT_BUILD_DOCS, that may be common to many sub-projects.

The cpputilities::Logger<> class provide basic facilities for supporting
logging operations. It comes with a cpputilities::SinkStdout class for
allowing logging on the standard output.

The cpputilities::Chronometer<> class provide basic facilities for
performing in-source time measurements. The commit also include the
skeleton for configuring library targets, tests and documentation
generation.

Add an empty README.md and a .gitignore file suitable for any C/C++
project developed with CLion.