config.in: cairo and ffmpeg checks

[hamish]

Hi,

we should finish adding Cairo support by building it into the ./configure script.

the attached patch also adds an extra test needed for ffmpeg.

I do not know much about autoconf and ./configure scripts so I don't claim that the attached patch will work, but it should be a good starting point.

Hamish

[hamish]

patch adding cairo support and more ffmpeg checks to ./configure.in

[glynn]

FFMPEG and Cairo changes

[glynn]

I have committed the changes in cairo.diff (plus the updated configure script, omitted from the diff for clarity).

That version uses pkg-config for the compiler and linker flags rather than --with-cairo-{includes,libs}, as that's the normal way to obtain that information, and adds the corresponding changes to Platform.make.in and the cairo-related Makefiles.