build_rpm.sh: set --build to guess for Darwin

Darwin platform is not RPM based, thus by default on Darwin we
pick 'quess' option to figure out system triplet. For other targets
default is 'auto', which assumes RPM based Linux distribution and
fetches system triplet from RPM directly.

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>
build_rpm.sh: bump zlib version to 1.2.8

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>