create source RPM cpack-way

when building binary packages, this source rpm will use
same BUILD_CONFIG and WITH_SSL values that were used when
creating the source RPM.

Only do it for a reasonably new cmake, where
source rpms are known to work (3.10.2 is ok, 3.5.2 is not).

And force a shorter CPACK_RPM_BUILD_SOURCE_DIRS_PREFIX so that
a source rpm could be built from a standard location in /usr/src