Improved file path compatibility

[benediktadams]

The line

source $(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" &>/dev/null && pwd)/${machine}/config_directories.sh

in rive-cpp/dependencies/config_directories.sh didn't work, when the library was cloned to a file path containing spaces.

By first writing the script_dir path to a variable and using that for the source call, we can support paths with spaces.