Mininum version of the dependencies

[jube]

For now, the minimum versions of the dependencies are based on Debian stable. Before gf 1.0, a new Debian stable (stretch) will be out with new version:

• SDL2 2.0.5
• Freetype 2.6.3
• Boost 1.62
• CMake 3.7.2

The question is: do we put these versions as a minimum?

The answer will probably be yes for SDL2, Freetype and Boost but no for CMake (we can rely on an older CMake version for building gf, but which one?).

[jube]

In the latest Ubuntu LTS (xenial):

• SDL 2.0.4
• Freetype 2.6.1
• Boost 1.58
• CMake 3.5.1

[jube]

In the latest Debian (buster):

• SDL 2.0.9
• Freetype 2.9.1
• Boost 1.67
• zlib 1.2.11
• pugixml 1.9
• CMake 3.13.4

[jube]

In the latest Ubuntu LTS (bionic):

• SDL 2.0.8
• Freetype 2.8.1
• Boost 1.65
• zlib 1.2.11
• pugixml 1.8.1
• CMake 3.10.2