Fixes #3694: Add external libevent hooks / add wrapper compiler RPATH support

NOTE: this work just recently hit the trunk, and is being committed to
the v1.7 branch very quickly (with very little soak time) because of
the compressed timeframe for the v1.7.4 release (with RM permission).

---svn-pre-commit-ignore-below---

r28479 [[BR]]
Fixes #376: bu default the wrappr compilers will enable rpath support
in generated executables on systems that support it.  Use
--disable-wrapper-rpath to disable this behavior.  See text in
README about --disable-wrapper-rpath for more details.

r29830 [[BR]]
Clean up variables used in configure with OPAL_VAR_SCOPE.

This is helpful in the work for #3694: ensure that many places that
eventually end up in configure don't overly-pollute the global shell
variable space (because debugging accidental shell variable pollution
can be a real pain).

Refs #3694

r29832 [[BR]]
cosmitic fix to remove redundant -lfca
use CPP extra flags var which propagated to coll/fca and scoll/fca
Refs: #3694

r29838 [[BR]]
Oops!  r29830 added a variable into OPAL_VAR_SCOPE that is actually
needed in the global scope ($ompi_check_fca_dir).  This commit removes
it from the OPAL_VAR_SCOPE, so it should be fixed now.

Sorry about that, folks!  :-(

r29841 [[BR]]
cosmetic fixed fpr hcol autotools
Refs: #3694

r29842 [[BR]]
Comment out the badness from the hcoll configury, originally from
r29830 -- Jeff will straighten this out with Alexander in person next
week (I can't test this myself because I have no access to libhcoll).
Sorry for the hassle...

Refs #3694

r29847 [[BR]]
revert Jeff`s fix, now hcol can be compiled with it
Refs #3694

r29848 [[BR]]
Addendum to r29847: actually remove the old OPAL_VAR_SCOPE_POP

Refs #3694

r29897 [[BR]]
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

r29898 [[BR]]
Add missing include, exposed in "external libevent" work.

Refs #3694

r29899 [[BR]]
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

r29901 [[BR]]
Fix rpath m4 typo that seeped in at the last minute.

Refs #3694

r29903 [[BR]]
Update the --enable-wrapper-rpath help string to be correct.

Refs #3694