[mosh-devel] Added rhel/centos

[Reini Urban]

Hi,

We only check for __builtin_bswap64 but not for the byteswap.h macros on 
rhel. I added it.
See https://github.com/keithw/mosh/pull/150

pipe2 O_CLOEXEC is also missing from older rhel.

centos5.8 boost-1.33 is too old. It doesn't have boost/typeof/typeof.hpp
1.34 is required at least

The fedora spec cannot be used since rhel/centos does not offer protobuf 
packages. But the reqs build fine from source.

Tested with CentOS release 5.8 (Final)
-- 
Reini