I've downloaded some library archived source files, extracted it in some-library directory, and then installed it with standard Autotools procedure as specified by library README file:
./configure
make
sudo make install
Is it now safe to delete some-library directory because library files are installed in the default directories unless otherwise specified?
Yes, after you compiled the libraries and installed the produced binary files to their destination directories, you can safely delete the source directory. Please note however that sometimes it is still handy to keep them around as they may contain additional documentation or usage examples - and there are occasions when you might have to recompile them.