it seems that you have to include gnustep-make package in your Sogo repo config
db yum_repositories set sogo repository \
BaseURL http://inverse.ca/downloads/SOGo/RHEL5/\$basearch \
EnableGroups yes \
GPGCheck no \
Name "Inverse SOGo Repository" \
IncludePkgs gnustep-base,gnustep-make,libmemcached,libwbxml,sogo*,sope49* \
Visible yes \
status disabled
note: not tested, should work, please give us feedback.. it it works, please edit wiki page accordingly (ask for a wiki access if you don't have any)