SCons improvements

1.  Implement changes in SCons to improve detection of ROOT installation
and SCons version checking (now checks for version 4.9.9 of SCons, which
is the newly implemented version number in the local SCons installation
(in podd).  Note that this change will now require use of that updated local version as
packaged with podd.

2.  Change to SConstruct.py to add -DHALLC_MODS defines (now consistent
with Makefile) for conditional compilation in certain parts of podd.

3.  Improvement in configuration for 64bit/32bit compilation on Linux.