cairo: fix build of any2ppm if no fork().

The test program any2ppm can run as daemon. This feature can be disabled
at compile time, if the required headers are not present. However the
support for fork() is not checked.

Fixes http://autobuild.buildroot.net/results/4f2/4f271d55d7060d412aa336cfc2da3be3538d3594/

.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>