Update the Mapserver installation documentation to reflect the move to cmake

[havatv]

Mapserver has migrated from autotools to cmake ( MapServer/MapServer#4617 ).
This change only seems to be reflected in the installation documentation as a warning in unix.txt that the documentation is not up to date.
At least unix.txt needs to be updated.
Please add other files that need to be updated.

[havatv]

@tbonfort, could you check if the update to unix.txt is OK?
Are there other files that need an update?

[havatv]

@tbonfort, I am eager to close this issue, so I will repeat my question from 2014:
Could you check that the update to unix.txt is OK?
Are there other files that need an update?

[tbonfort]

I just had a quick look, thank you for your additions. Here's a few things that would need updating:

• GD support has been removed
• SDE support has been removed
• RSVG can be used as an alternative to CAIROSVG
• HARFBUZZ support should be added (requires FRIBIDI and vice-versa)
• Libtiff/libgeotiff has been removed

[havatv]

RSVG was already mentioned. In the latest commit I have tried to address the other issues. Please have a look, @tbonfort.
In the INSTALL.CMAKE file, SDE support is still described. Is that a mistake?