test: fix build of any2ppm if fork is not available.

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.

This patch fixes this issue.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>

Index: cairo-1.12.10/test/any2ppm.c
===================================================================
--- cairo-1.12.10.orig/test/any2ppm.c	2012-04-19 14:17:34.000000000 +0200
+++ cairo-1.12.10/test/any2ppm.c	2014-04-25 11:13:30.000000000 +0200
@@ -79,7 +79,7 @@
 
 #include <errno.h>
 
-#if HAVE_UNISTD_H && HAVE_FCNTL_H && HAVE_SIGNAL_H && HAVE_SYS_STAT_H && HAVE_SYS_SOCKET_H && HAVE_SYS_POLL_H && HAVE_SYS_UN_H
+#if HAVE_UNISTD_H && HAVE_FCNTL_H && HAVE_SIGNAL_H && HAVE_SYS_STAT_H && HAVE_SYS_SOCKET_H && HAVE_SYS_POLL_H && HAVE_SYS_UN_H && HAVE_FORK
 #include <fcntl.h>
 #include <signal.h>
 #include <sys/stat.h>