zeromq: disable test_fork when fork() is no available

zeromp's configure script correctly detects the fork availability, but
unconditionally build test_fork program whatever the fork() availability.

This patch fixes the build-system by disabling test_fork when fork is
not available (e.g. when !BR2_USE_MMU).

Fixes:
  http://autobuild.buildroot.org/results/359/3599cc3b7bf2bb22a78961cd84d21cb03cbd7015/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
[yann.morin.1998@free.fr: remove Makefile.in hunks]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr: tested against the failed bfin config, as well as
 a i686 target]
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>