btl/vader: add support for SGI's implementation of xpmem and add support
for 32-bit architectures.

This commit also modifies _OMPI_CHECK_HEADER to use AC_CHECK_HEADERS instead
of AC_CHECK_HEADER. This allows components to check for multiple headers
instead of just one. The new semantics of the header check in OMPI_CHECK_PACKAGE
are to return success if at least one of the specified headers exists. The new
semantics will not break current usage.

cmr=v1.7.5:ticket=#4053
coll/ml: fix some warnings and the spelling of indices

This commit fixes one warning that should have caused coll/ml to segfault
on reduce. The fix should be correct but we will continue to investigate.

cmr=v1.7.5:ticket=#4158