<infiniband/driver.h> cannot be included because it will fail to
compile. So per advice from Roland, in that case, just put manually
include the one ibv_*() prototype that we need. Use an undocumented
feature of AC_CHECK_HEADERS to check for <infiniband/driver.h> that I
was told on the autoconf mailing list (see the comment for more
details).
This commit was SVN r19857.