modified LDFLAGS to match cplex install more easily

The folder pointed to by the LDFLAGS variable does not exist in my install,
changed the ARCH variable so it contains the complete directory name.
My vars.in file looks like this now, which works:

CPLEX_DIR = [path_to_cplex]
ARCH = x86-64_linux#32bit: x86, 64bit: x86-64
GPP = g++