[G12] switching build_system to 64bit development branch.

[John T. Goetz]

for your convenience I have created a branch of the build_system where
the -m64 flag is used instead of the -m32. This can be used by doing the
following steps:

1. cd into the build_system (trunk) directory. It could be in
clas6-trunk/build_system or you may have it elsewhere
like /usr/local/lib/python/build_system
2. issue the command:

svn switch
https://jlabsvn.jlab.org/svnroot/clas/build_system/branches/64bit

3. do 'svn up' inside that directory.
4. build a1c normally.