Fixed the way that configuration files are found and used, searching
using the following order of precedence:

1) The configuration file specified on the command line invocation of
the compiler
2) A file with the appropriate default name for the system (usually
shadow-linux-64.xml or shadow-windows-32.xml) in the directory
containing the first .shadow file to be compiled
3) A file named shadow.xml in the directory containing the first .shadow
file to be compiled
4) A file specified by the SHADOW_CONFIG environment varible
5) A file with the appropriate default name for the system in the
directory where shadowc is located
6) A file named shadow.xml in the directory where shadowc is located

Once a configuration file has been found, the system import and imports
specified by it are relative to the location of the configuration file,
unless they are absolute paths.