Improve Makefile to set `CHPL_HOME` if unset

[bmcdonald3]

In a comment from Brad , it was suggested that having the Makefile set CHPL_HOME if it is unset using chpl --print-chpl-home would be an improvement to the build process, since we are relying on CHPL_CXX being set to build the C++ code.

Since this flag was only added in 1.31, we'd like to add that after 1.30 support is dropped.