This patch simplifies driver/xf86-video-ati/src/Makefile.am somewhat.

If you think this is desirable to have, I can probably produce similar changes
for the rest of the tree.  Note that this change causes a considerable reduction
in Makefile.in size and complexity.

Please note that
var =
var += foo
var += bar

is equivalent to
var = foo bar

and as such just a matter of taste, which one should be used,
except that, when combined with Automake conditionals, order may not be
stable in the output; this is fixed in Automake 1.9.6.  This fact has no
bearing to this patch, but may have impact to further similar changes if
desirable.

Cheers,
Ralf