rpm: fix "configure: WARNING: invalid host type"

This commit fixes the above warning when "make rpm". This happens
if you ./configure without --enable-zookeeper or --enable-sheepfs
before "make rpm". This is because %{fuse_configure_args} and/or
%{zookeeper_configure_args} are not expanded at configure step
during "make rpm".

Now I set each of them to %{nil} as default if the corresponding
./configure option was not given.

Signed-off-by: Takashi Menjo <menjo.takashi@lab.ntt.co.jp>

Author: tmenjo

sheepdog.spec.in

@@ -6,9 +6,13 @@
 # Configure args
 %if 0%{?enable_fuse} == 1
 %define fuse_configure_args $(echo "--enable-sheepfs")
+%else
+%define fuse_configure_args %{nil}
 %endif
 %if 0%{?enable_zookeeper} == 1
 %define zookeeper_configure_args $(echo "--enable-zookeeper")
+%else
+%define zookeeper_configure_args %{nil}
 %endif
 
 Name: sheepdog