configure.ac:  Remove -rc suffix from the configure version number.

Remove the rc suffix since RPM complains about.  For rc release
builds the "rc" in the git describe string is suffcient for
everyhting but RPM.  For rc release builds (i.e. not gitbuilder)
add a flag to the spec file.

Signed-off-by: Gary Lowell  <gary.lowell@inktank.com>