Fixes #707

With rpms, if `--architecture` and `--rpm-os` are specified and
different than the host, `--target` would be passed to `rpmbuild` twice,
causing an error.

Example: an OS X host (`uname -s` = "Darwin") with x86_64 architecture
building for `--rpm-os linux` and `--architecture noarch`.

This change only adds `--target` to the `rpmbuild` args once, fixing
this specific error.

lib/fpm/package/rpm.rb

@@ -309,12 +309,16 @@ def output(output_path)
     args = ["rpmbuild", "-bb"]
 
     if %x{uname -m}.chomp != self.architecture
-      args += [ '--target', self.architecture ]
+      rpm_target = self.architecture
     end
 
     # issue #309
     if !attributes[:rpm_os].nil?
       rpm_target = "#{architecture}-unknown-#{attributes[:rpm_os]}"
+    end
+
+    # issue #707
+    if !rpm_target.nil?
       args += ["--target", rpm_target]
     end