btl/vader: silence warning

correctly check HAVE_SYS_PRCTL_H
Add new PML yalla, which uses mxm directly to reduce overhead.

http://starwars.wikia.com/wiki/Ubed_Yalla
One more fix for libltdl configuration.
Remove unneeded "-I" statements from conftest compilation command.
btl/vader: fix compilation on OS X
Merge pull request #246 from yosefe/topic/pml-yalla

[RFC] Add new PML yalla, which uses mxm directly to reduce overhead.
pmix: correctly split pmi messages