Restructuring code w.r.t. configurations and definitions:
- #defines have been mostly removed from configurations
- a dedicated config file has been added for configurations with constexpr
- past configure file is now only a cmake wrapper
- wrapping global functions into namespaces

Removed all adafs and ifs occurrences. Now called gkfs

 - improved statfs system call
 - intercepted statvfs library call

Both of the above function now report the aggregated real size
of all the data-nodes.

You can test this with `df "/tmp/gekkofs_mountpoint"` command

`tar` and other gnu coreutils are using __openat{64,}_2 variant.
We need to intercept also that apart of the main `openat` function.

StackOverflow related question [1]

[1]: https://stackoverflow.com/questions/9161116/intercepting-the-openat-system-call-for-gnu-tar

In the glibc both readdir and closedir functions marks the @dirp parameter
with a non-null attribute.
If we use a compiler optimization level grater then 1,
the `dirp == nullptr` expression is evaluated to false
at compilation time and the relative if blocks is cutted out
of the final binary.

In order to avoid this bheaviour, we implement a functions and then set
that as weak alias of the real glibc function call.