cmake integration

[wllm-rbnt]

This PR replaces make by cmake as a build system for the mksquashfs and unsquashfs tools.
User defined variables & options are available in the top section of the CMakeLists.txt file.
LZMA1 support is not implemented.

It works with ninja:

$ sudo apt install ninja-build
$ cd squashfs-tools/squashfs-tools
$ cmake -B build -G Ninja
$ ninja -C build
$ ninja -C build install

or make:

$ cd squashfs-tools/squashfs-tools
$ mkdir build
$ cd build
$ cmake ..
$ make
$ make install

This PR should fix #256