detect 64bit processors and add -fPIC

this is an intermediate solution as I probably can use ADD_DEFINES()
to add the command line options for the compiler instead of fiddingly
with CMAKE variables

Props to Brandon for finding the solution in an openwengo patch.
http://dev.openwengo.org/trac/openwengo/trac.cgi/changeset/9139
Long Live CMake!