2 fixes for "make dist"

@sayantansur @jithinjosepkl Please review.  These two commits are necessary to fix "make dist", as noted in #219.
I think it's 6 of one, half dozen of another.

The Automake manual says (section 8.1.1):

> Header files listed in a _SOURCES definition will be included in the distribution but otherwise ignored. 

We typically list them in SOURCES in OMPI because they *are* sources.  We typically use EXTRA_DIST for non-code files (e.g., README, LICENSE, etc.).