cmake: redesign scylla's `CMakeLists.txt` to finally allow full-fledged building
cmake: redesign scylla's `CMakeLists.txt` to finally allow full-fledged building

This patch introduces many changes to the Scylla `CMakeLists.txt`
to enable building Scylla without resorting to pre-building
with a previous configure.py build, i.e. cmake script can now
be used as a standalone solution to build and execute scylla.

Submodules, such as Seastar and Abseil, are also dealt with
by importing their CMake scripts directly via `add_subdirectory`
calls. Other submodules, such as `libdeflate` now have a
custom command to build the library at runtime.

There are still a lot of things that are incomplete, though:
* Missing auxiliary packaging targets
* Unit-tests are not built (First priority to address in the
  following patches)
* Compile and link flags are mostly hardcoded to the values
  appropriate for the most recent Fedora 33 installation.
  System libraries should be found via built-in `Find*` scripts,
  compiler and linker flags should be observed and tested by
  executing feature tests.
* The current build is aimed to be built by GCC, need to support
  Clang since we are moving to it.
* Utility cmake functions should be moved to a separate "cmake"
  directory.

The script is updated to use the most recent CMake version available
in Fedora 33, which is 3.18.

Right now this is more of a PoC rather that a full-fledged solution
but as far as it's not used widely, we are free to evolve it in
a relaxed manner, improving it step by step to achieve feature
parity with `configure.py` solution.

The value in this patch is that now we are able to use any
C++ IDE capable of dealing with CMake solutions and take
advantage of their built-in capabilities, such as:
* Building a code model to efficiently navigate code.
* Find references to symbols.
* Use pretty-printers, beautifiers and other tools conveniently.
* Run scylla and debug it right from the IDE.

Signed-off-by: Pavel Solodovnikov <2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>
Message-Id: <20201103221619.612294-1-2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>

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