Just like the CFLAGS variable, the LDLIBS variable should be
extensible with environment variables.
This is for example required because Debian packages (should be)
built with special hardening flags:
https://wiki.debian.org/Hardening
For various enviroment it is very valuable to have externally
overwriteable versions (such as distribution packaging, where
a maintainer wants to set a specfic version).
Also, not all build enviroments have git available (e.g. Debian),
therefore the expression `$(shell git describe)` would be evaluated
to ''. This patches makes the version variable used in the Makefile
overwriteable with an environment variable.
The Makefile builds object files in src/ and src/intern_blks/,
however the clean target only deleted object files inside src/
Now the clean target deletes object files in src/, src/intern_blks/
and the program itself
