README: Correct env variables to build using specific libparted version

CPPFLAGS - Needed to instruct the compiler to use libparted include
files, <parted/parted.h> etc, from the non-default location.

LD_LIBRARY_PATH - Not needed to specify an additional directory to find
libparted shared libraries at run time, as the linker build the
additional directory into the executable based on the LD_RUN_PATH
environment variable.