Install fails when invalid C++ compiler is given

[jpkenny]

Deficiencies in existing configure:

1. AC_PROG_CXX is run in before the actual compiler used is determined during SST_CORE_CHECK_INSTALL()
2. An invalid compiler given via CXX will fail tests, but not cause a fatal error during configure. Libtool is put in an incorrect state and the errors don't occur until install time and are very hard to track back to the root cause.

Example error:
install: .libs/libfabric.0.dylib: No such file or directory

To reproduce:
Specify invalid compiler via CXX