Bug#888694: xserver-xorg-video-mga FTCBFS: abuses AC_CHECK_FILE for build system files

[Helmut Grohne <helmut@subdivi.de>]

Control: clone -1 -2
Control: reassign -2 src:xserver-xorg-video-tdfx
Control: found -2 1:1.4.7-1

On Sun, Jan 28, 2018 at 08:08:26PM +0100, Helmut Grohne wrote:
> xserver-xorg-video-mga fails to cross build from source, because it
> abuses AC_CHECK_FILE for finding headers. AC_CHECK_FILE is supposed to
> find files on the host system. You cannot usually expect the host system
> to have headers, so this use is incorrect. It happens to also break
> cross compilation. To check for files on the build system, simply use
> "test -f". The attached patch implements that. Please consider applying

The same issue occurs with xserver-xorg-video-tdfx and the same patch is
applicable here.

Helmut