xpmem/config: simple xpmem search on Cray's

Use the pkg-config related m4 functions to find out where
Cray's xpmem.h and libxpmem are located on a system.

With this commit, there is no longer any need to have to
explicitly indicate an xpmem install location on the configure
line, at least for Cray systems running CLE 4.X and 5.X.