From xserver 1.6 on, the xf8_32bpp module no longer exists. However, mga still uses it and thus cannot be built against 1.6.
This was the commit:
And this was the bug:
Seems that 1.4.x of course already has this, so I guess 1.9.100 needs to be updated.
This seems a relevant commit:
Mass closure: This bug has been untouched for more than six years, and is not
obviously still valid. Please reopen this bug or file a new report if you continue to experience issues with current releases.