Add missing argv.h includes.

Noticed these as part of #3694: external libevent's don't cause argv.h
to automatically get included.

Refs #3694
Add missing include, exposed in "external libevent" work.

Refs #3694
This commit combines 3 somewhat-unrelated things, which unfortunately
got linked together (work on one caused work in the other):

 * Clean up a bunch of VAR_SCOPE issues in configure.  This includes:
   * Using VAR_SCOPE_PUSH and VAR_SCOPE_POP in more places
   * Cleaning up the use of some shell variables (e.g., name them better)
 * Add support for external libevent via
   --with-libevent=<dir-to-libevent-install-tree>, as specifically
   asked for by downstream packagers.
 * Revamp how wrapper compiler RPATH (and RUNPATH) support is done.
   The external libevent work exposed weakenesses in how the original
   RPATH/RUNPATH work was done, so we had to re-do it to be a bit more
   robust.

This work has not yet been tested on Solaris.

Refs #3694
Fix various items discovered by review of ticket #3951.
Fix rpath m4 typo that seeped in at the last minute.

Refs #3694