Use CMAKE_CURRENT_SOURCE_DIR instead of CMAKE_SOURCE_DIR

[traversaro]

This simplifies the use of the library in an downstream CMake project using the FetchContent CMake module (https://cmake.org/cmake/help/latest/module/FetchContent.html).

[markaren]

Before I merge this, can you explain the difference between the variables?

[traversaro]

• CMAKE_SOURCE_DIR : The path to the top level of the source tree.
• CMAKE_CURRENT_SOURCE_DIR : The path to the source directory currently being processed.

See https://cgold.readthedocs.io/en/latest/tutorials/cmake-sources/subdirectories.html#source-variables for more info.

Now they correspond, but if I embed FMI4cpp in my project, using FetchContent, git submodules or even by copy-pasting it, and if I add the FMI4cpp to the build with add_subdirectory, CMAKE_SOURCE_DIR will now refer to the root directory of my project, not anymore to the root directory of FMI4cpp, so list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake") will not have its intended behavior.