gitignore: a few more consolidations
libfabric: do a proper check for intrinsic atomics
libfabric configury: make AC_CONFIG_FILES be unconditional

Also add the generated config.h file to .gitignore.
libfabric configury: add more tests

Properly test for some dependent libraries; don't just assume
elsewhere in Open MPI's configury will find those libraries.  Also
consolidate some CPPFLAGS and clarify some comments.