Applied an RPM building patch from Malte Starostik.
* changed boost-jam-3.1.3.src.tar.gz to boost-jam-3.1.3.tar.bz
* Added a variable RPMARCH to Jamfile.
* remove the .tar.gz and .tar.bz2 before recreating them. gzip asks for
confirmation otherwise, bzip2 just dies.
* Get rid of hardcoded /usr/src/redhat/... as it's a) distribution-specific
and b) a user can override the build root to use.
[SVN r15916]
Vladimir Prus