- use officially supported pkg-config instead of pkgconf replacement
- add zlib library to dependencies
- remove GLU library which is not needed at all since this commit: 60dc171700
- add libgl1-mesa-dev and mesa-common-dev which contains OpenGL headers that we need
- use libcurl-gnutls instead of openssl because it is used by default in Debian/Ubuntu for STK and many other projects.
* correct spaces
* proper names with a capital letter
* also use block code indentation for single-line commands to keep a consistent look (`this code style` should only be used for code/commands mentioned inside of other text)
