xplat_config.h and mrnet_config.h not installed with Lightweight MRNet

[bwelton]

When building lightweight MRNet by itself (via "make lightweight-all") the files xplat_config.h and mrnet_config.h are not installed onto the system when Make Install is called. Thus if you are building MRNet with only lightweight support then you would not have these required headers installed on the system.