[librdmacm,5/8] configure: Use automake's option "subdir-objects"

[librdmacm,5/8] configure: Use automake's option "subdir-objects"

Following advice in "Autotool Mythbuster" [1], option subdir-objects
can be used to have Makefiles create object files in the same
directory than theirs source files.

It reduces clobbering in the build directory.

[1] "Autotool Mythbuster", by Diego Elio "Flameeyes" Petten`o
http://www.flameeyes.eu/autotools-mythbuster/automake/nonrecursive.html

Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Sean Hefty <sean.hefty@intel.com>